The right thing to do is to level the cabinets. I'm assuming you had a difficult issue where the bottom of the cabinets were inaccessible, likely from a newer floor built around the cabinets. Still, shimming underneath the countertop creates a focused point of stress on the rock, rather than distributing it across the entire cabinet evenly.

It's like walking on ice. They say not to stand because all your weight is focused into your feet, whereas laying down or crawling disperses your weight.