Using the same finish everywhere is ideal, but not mandatory. It would be more important if you were finishing an unrestrained panel like a cabinet door, but for a countertop (which is screwed to the cabinets), you don't need to be quite so retentive. I think poly on the bottom will be fine.

Recommendations that all sides should be sealed the same way are idealistic statements of principle, not practical necessity. The ideal is that all sides of a chunk of wood should be exposed to identical environmental influences, but in most cases this is an impossible goal. The top and bottom of any table-like surface are never going to get the same amount of exposure to water or direct sunlight, for example.

It's far more important that the wood be at an appropriate moisture content when the countertop is manufactured, and that it is stored properly (i.e. not in the garage or basement) before installation.

The coating only slows the movement of moisture in and out of the wood as the humidity changes over the seasons.

The wood WILL move, and it will simply break the fasteners or the cabinets if it moves enough.

Old proverb/aphorism: as above, so below.
Casey

I would expect that an oil finish will need to be renewed at some point. You will not be able to do that to the underside - a different finish may be warranted.

If you have an oil finish on the top, it will have nearly zero effect on humidity transfer.

Here are some articles on the subject
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Here is a link that might be useful: See the first few lines on table 15-3

That's a handy document, Bob.

At the risk of making the OP's decision harder, It's worth pointing out that many wood finishing products sold as "oil finish" are really thinned varnishes, and so perform better than one might guess from looking at that chart.

Also, the idea that one should treat all sides of a board the same way is meant to make sure the piece doesn't cup as it gets (subtly) larger or smaller, not to prevent movement altogether.

Yes, "oil" finish can mean anything such as BLO, True tung oil, thinned varnish, oil-varnish blends, mineral oil, mineral oil and paraffin.

If linseed oil comes from squeezing the seeds of the linen (flax) plant, where does Danish Oil come from? Squeezed Danes?

Thanks guys! I'm probably going to use Waterlox w/ oil stain mixed in (stain in the first coat only) then more coats of the waterlox. I haven't used their product, but apparently a lot of people use them for countertops, with good results and the monthly touch-ups don't require sanding between coats!

I assume I should make some corner brackets in the cabinets so I can screw up into the 1 1/2" thick tops- liquid nails is not enough to hold them, right? And the holes in the brackets should be elongated for movement also?

Yes to screws through corner brackets, yes to elongating the holes to allow for movement. DO NOT use liquid nails, as this stuff will cause the moving countertop to damage your cabinets.

Thanks jon1270! OK, no liquid nails, what's best to use, plain silicone- The bottom will be poly'ed when I install, I assume silicone's the way to go? Just some good wood glue and clamps for any seams right? (I'll sand off any poly on those edges before gluing, or tape it first)

Thanks, didn't realize that about the liquid nails, doesn't allow for movement I guess?

Where do you want to use silicone? You don't need any adhesive between the cabinets and countertop.

Can't say how you should handle joints / seams since this varies by circumstance. Some can be glued, some can't.