I'm surprised there isn't a "Always tip but a variable amount" option.

Which is what I am. I suppose I might not tip if the witstaff pissed me off. But they know they're working for tips and I've never had a bad situation.

Sometimes I'll also tip higher if I know I or my wife is being a pain (my wife is a bit picky so sometimes I'll have to work the poor waiter through a whole list of substitutions).