A friend writes:
The base tip rate is 18 percent. Cash, please. If you use a credit card, you must tip more, because many servers don't report cash on their income tax, so cash is worth twice is much to them. 15 percent is what you tip when the service is just barely adequate. 10 percent -- you'll never be served again.
That's an interesting viewpoint, but it's not a practice I'm inclined to agree with, quite. We always put our tip on the credit card, and never feel inclined to tip extra because we are tipping that way. I see no obligation to tip extra because I'm not letting someone cheat on their taxes, nor to help them cheat.
I prefer to tip closer to 20 percent of the post-tax bill than to 15, and under no circumstances will I tip less than 15. I've been known to give people, including my dear old mother, tremendous grief for wanting to tip less than 15. (Money's really tight right now, so we're not eating out at all, which renders the whole issue moot, to be honest.)
Honestly, my inclination is to tip a little extra when we get shoddy service, although I'll also report it to the manager. A lousy waiter or waitress may not deserve the extra tip, but there's a lot of things I haven't deserved that I've still received, by God's grace.