Skip the miter if you want the joint to stay tight; it will open no matter what you do.

Countertop installers use a set of bolts that are inset to the underside of the counters when they want to keep a 45 degree miter together. I've seen these used with plywood and particle board substrates, and I think they would work fine for butcherblock. You typically just drill a flat bottom hole on each side of the joint so that the bolt spans the joint and is perpendicular to the line. You put the bolt in place and tighten the special nut, and it draws everything together nice and tight. Two or three of these gizmos should do the trick. However, it would also be a good idea to apply glue to the joint before drawing it together with the bolts, and even a couple of biscuits would be good to keep the top surfaces aligned.

Here is a link that might be useful: Tite Joint Fasteners

The problem is the seasonal expansion and contraction of solid wood, which effectively changes the angles of the miter cuts. The joint will open on the inside as the wood shrinks, or on the outside when it expands. Fasteners and glue can only hold it together by forcing the movement to happen elsewhere , i.e. one leg of the ell (or both) move away from the wall, or the butcherblock cracks.

+1 for Jon's analysis.

jon-
In my experience, the amount of seasonal movement will depend on a lot of things, including temperature and humidity variataions, and the quality of the butcher block. I did a little Googling and these are the Fabrication Recommendations for John Boos Butcher Block Counter Tops:

1. CORNER JOINTS
All edges to be joined together must be true (exact). The recommended corner joints are MITER joints or fully BUTT
joints. With miter joints on boards of unequal width, a true miter usually gives the best result as any movement is
equalized, and the laminates will be close to matching at the joint.
1.1: Biscuit or tongue the joint. Joints should have "Lamella" biscuits or plywood slip tongues fitted
1.2: Bolt clamp the joint. Do not use towels. Fit bolt clamps (miter bolts) to the underside of all joints: 4" from the
front, 6" from the back of miters and 1½" from both sides of butt joints. On wide joints fit an extra clamp midway
between the others.
1.3: Butt Joints with opposing grain direction are never to be glued, instead use silicone.
1.4: Miter joints and butt joints with grain running in the same direction should be glued.

As the link points out, you can have problems with expansion/contraction if you screw down and/or glue down butcher block counters to the framework, but miters seem to be fine because the movement along the joint should be uniform on both sides.

Here is a link that might be useful: John Boos Installation Guide

A miter is an unstable joint. The 45* angle becomes a 44* or 46* depending on changes in humidity. If you try to lock the miter together, it will pull away at the wall, or the biscuits will fail. Like Scotty said on Star Trek "I cannae break the laws o' physics". If you place your trust in the sanguine claims of a manufacturer, and it fails anyway, remember they always have an "out" written somewhere in the fine print.
Casey

It's not very hard to find photos of finished installations with miters. I guess reasonable people can disagree.

Here is a link that might be useful: Photos

"It's not very hard to find photos of finished installations with miters."

That means nothing about how they will survive.

Wood is an anisotropic material.

It change size and shape in differing amounts depending on how the grain is oriented.

See Figure 3-3 in the Wood Handbook linked below.

Here is a link that might be useful: USDA Wood Handbook, Chapter 3

Think of the miter cut as a triangle. One leg (the length, parallel to the grain) moves little to none, the other leg (the width, across radial or tangential surfaces or somewhere in between, depending how the boards are cut from the log) changes in width a few percent. For a 24" wide countertop, this could be 1/4" or more. The result is the effective angle of the miter joint changes as is said above.

The fact that they tell you not to glue end grain to side grain admits that the wood is going to move in width.

>but miters seem to be fine because the movement along the joint should be uniform on both sides.

--------

Got any close-ups?
>"It's not very hard to find photos of finished installations with miters."

"If you place your trust in the sanguine claims of a manufacturer, and it fails anyway, remember they always have an "out" written somewhere in the fine print."

John Boos has been in business for at least 130 years making nothing but butcher block related products. In essence, they've forgotten more about how to put butcher blocks and countertops together than you'll ever know in your lifetime. The idea that they would purposely mislead someone on how to install THEIR products is ludicrous - the constant Internet skepticism about everyone being dishonest except the person making the post is tiresome.

Butcher blocks are end grain facing the top and bottom of the block. The pieces making up the butcher block are relatively small in size, and have been oriented against each other in different directions to offset expansion / contraction to stabilize the butcher block. That's why you find butcher blocks that are 50 - 100 years old that are still in one piece.

If you make a 45 degree miter, you are fastening edge grain to edge grain - not end grain to end grain. Wood expands and contracts with relative humidity the most on the end grain, not the edge grain. That's why table tops are fastened so that the wood can move over the length of the board and you can GLUE the edge grain together. In making cabinet doors, you glue the wood making the panels together on the edge grain over the length of the panel, and put space balls in the rails to compensate for the panel expanding and contracting over the LENGTH of the panel - you don't have to worry about the width as that is face and edge grain..

The fastening method recommended by Boos is the correct way to fasten the joint. The butcher blocks are not monolithic (once piece) and, therefore, DO NOT move like a single piece of wood.

For butcher blocks, I find that Mahoney's polymerized walnut oil (the liquid - not the walnut oil wax) makes the best finish. It will slowly build up in the wood as it hardens (it's polymerized) and will totally seal the surface. The surface is easily cleaned with soap and water. To maintain the surface a wipe with oil 1-2 times per month is all that is needed once the surface has been established.

A benefit of the oil is that if cracks do appear, the oil will penetrate the crack, swell the wood back together, and seal the crack. I have a John Boos block that is about 15 years old, crack free, and sealed with Mahoney's walnut oil.

You can order the oil directly from Mike Mahoney, or get it at outlets like Woodcraft Supply. Mike is a bowl maker and developed the finish for his own work. It is non-toxic, and easy to use.

Here is a link that might be useful: Mahoney's

The OP _never_ stated that he was using end grain BB. Kudzu9's installation manual quote does not specify end grain BB. Boos in fact sells both edge grain and end grain blocks.
Thanks for bringing it up at this late date. We all need to be confounded from time to time.
It's encouraging to know that Boos warrants their product no matter how it was installed, and under any conditions of use and maintenance, or the lack thereof. Seldom will a manufacturer give an unconditional warranty, so Boos is to be greatly commended, and thanks for making this claim for them, I'm sure they appreciate the positive press.
Casey

Thanks for the insights, swines and sombreuil_mongrel. I'm all for a good dialogue, but it's sometimes annoying to simply present a possible option and feel like you walked into an unmarked shooting gallery. Hopefully, the OP now has enough info to make an informed decision.