I confront this question every time I pick up teriyaki takeout from our neighborhood place and pay with a card. The chit is handed over with a prominent line to add a tip. And I usually do, but only $2. or so on a bill of between $20 and $30, so it's sort of a minor thank-you to the poor kid behind the counter, whom I don't think is making very much in that job. Maybe I should X it out, but guilt gets the better of me... and this is not The Four Seasons, exactly.

Interesting that when you pay with cash the question just doesn't arise.