I recently had a 3cm granite countertop installed over a plywood base (approx. 72" x 24"). As is often the case, the plywood thickness varies throughout by 0.010" to 0.050". Thus, even though the base is quite level in both dimensions, there are random gaps between it and the granite. I'm concerned about the potential for cracks in the future, esp. if someone puts something heavy on the granite (or climbs up). My biggest concern is one end of the granite, where the last 12" or so floats over the plywood by 0.010-0.030". For now I shimmed this area with some thin metal plates.
Of course, this does nothing for any other hidden gaps; eliminating these would probably require a very thin layer of thinset or similar, then bed the granite on top while wet. After checking various resources, I haven't found anyone who actually does this (other than tile).
This is my first experience with granite. Am I being overly-concerned, and the granite will likely flex without issue if pressed? Or is this a common problem that most people ignore & hope for the best?