Your question has already been answered, but just let me say that you are not obligated to leave any more tip regardless of how good a deal you've gotten or than it's ONLY 18% - servers never expected anything more than for us to sign the bill. When it was a 15% tip, I did indeed leave more for good service but at 18% of what tends to be a very high bill (because of the appetizer, entree, and dessert for each person) I felt no obligation to leave more.
For instance at le Cellier for two people on our bill, the waitress received a $19 tip.