When we go to Ryan's we always pay with a credit card and they ask us if we'd like to put the tip on it, but you pay BEFORE you eat there because it's an all-you-can-eat buffet. We're always like, "No, we'll leave cash when we see what the service is like." I would NEVER eat at a forced tipping establishment except for Waffle House. They add a forced gratuity onto take-out orders, but since I'm not staying for service, that doesn't bother me too much. As long as they get the order right, I'm happy. And I check it before I leave.
Oh yeah, we always leave something even if the service is horrendous. We leave at LEAST 10% even to the really bad ones. We leave 20% and occasionally more for good service.