I always check my bills. With computerization there are fewer math errors than there used to be--but charging the wrong price, item or number of items does still happen. I cannot help but notice that the error is almost always in favor of the restaurant--though I point out the error either way.
I end up feeling that this is often deliberate --and yet I have continued to tip at my normal rate on these bills. I realize adjusting down runs the risk of punishing the waitstaff for a truly inadvertent error (or one out of their hands entirely) but not adjusting down rewards the guilty (who probably reap higher tips from all those people like my father-in-law who never check a bill).