I am idly curious about something that hopefully anybody out there who has worked as a waiter or waitress can clue me in on.

My understanding of tipping protocol is that when you use a gift certificate or discount coupon at a restaurant, you tip on the full amount, not the discounted amount. Seems obvious to me. But when I wonder how many people calculate the tip based on the discounted amount. Sometimes this can be a pretty drastic difference. At Newick's, a local seafood restaurant and site of an RU2 meal, they take $10 off your total bill if it's your birthday. That's usually enough to cover my entire meal and some of Leen's. So when the bill comes, it might be something like six or eight dollars. I can imagine how people who aren't very conscientious would figure 15-20% of that amount and call it good.

My question is, how often does this happen? Do serving people get an inward sense of foreboding when they learn that a discount deal is to be used, or are most people good about tipping on the full amount?