I cut 6 layers at a time, really I think it just calls for pratice. You have to have pressure on the ruler, and the cutter must be straight. All my squares come out perfect. I have been known to cut a layer of 8 when I am in a hurry. I don't use a big cutter, just a simple one.
It does take a bit of pratice, and a steady hand. Start with what your comfortable with, don't do more than your able to.