I don't want my waiter or waitress to be taxed on the tip I leave them, either. And, I also don't want to put them in the moral dilemma of whether or not to report it on their tax return. So, instead of leaving a cash tip, I tell them to "Buy low and sell high."
The essential American soul is hard, isolate, stoic, and a killer. It has never yet melted. ~ D.H. Lawrence