True, but the waiters and staff are the public face of your restaurant. The owner/manager doesn't get involved in every meal they serve. If you suspect your staff will get cranky with Groupon customers, you need to fix that. I don't care if that means firing the worst offenders and making it clear that you won't tolerate such behavior, or if it means ahead of time declaring that you're going to "fix up" bad tippers behind the scenes by supplementing with the restaurant's cash. It really doesn't matter to me how they resolve the situation, but the owner of the restaurant is responsible for giving his/her customers the best experience possible.