Perfectly level. Shims are your best friend, and screw in at the end. I think.

100% level in all dimensions is required.

DH checking in here: I recommend checking with the counter top company to see if they have specs. In preparing for granite in our kitchen, somewhere I read that there should be no more than 3mm of deflection over 10 ft. If there is more deflection than that, the installer would need to excessively shim the counter top. I don't know if these specs would apply to your counter type or not.

We had the assistance of an experienced installer, and we tied together cabinets with their tops aligned to each other, and then shimmed the group to make the assembly flat and level before securing the cabinets in place.

Cabinets need to be 95% or so level. Natural stone varies in thickness, so even dead level cabinets will need shims to level tops.

Rip some regular plastic laminate and some vertical grade (V-32) into 1" strips for perfect non-compressive shims. Hot melt them into place on cabinets or slicone and snap 'em off flush after the tops are on.

Thank you for all your advice. I'm not sure how to even measure this - we have a 4-foot level, and on that, it looks great, and with very few places that I see "daylight" under the level on the cabinet edges. But when I run my finger at the places where cabinet edges butt up to each other, they are a bit off. Maybe its time to break out the laser level again.

Am hoping to talk with my counter guy this week.

I may not be able to describe this well, but I'll give it a shot.

We were able to check the flatness of our assembled cabinets by pulling reference strings across the surface. We tapped small nails vertically into the cabinets at some edge locations, leaving most of the nail sticking up. We then pulled strings (we used mason's string) tight between the nails, so the strings ran the length and width of the cabinets. We used a small wood block (about a 1/4 inch thick) to set the gap between the cabinet and the string at each end (right near the nail). Then with the string the same height at each end, we could use the block to check the clearance at all the places in between the nails.

This worked great for us where we needed to make sure the entire u-shaped set of cabinets were in the same plane for the granite.

Wow, that sounds doable! Thanks and I will give it a try.

Juno_barks:

The August, 2014, issue of The Journal of Light Construction has an excellent article on setting cabinets and I highly recommend it.

I did have one quibble and I wrote the editor about it. The article mentions the importance of keeping cabinets plumb, level, and square, but failed to mention the importance of keeping them in plane to each other.

Hi- I know this is an old thread but is it going to be a problem if my island is 1/16th (maybe 1/8th but looks like less) tilted in one direction? I have a mitred edge so there are 2 layers of plywood underneath the quartz. Any advice?

1/16" to an 1/8" out of level is not level. Period.