CC- practices are different in different countries. In the USA, you are not perking up someones salary, but rather their wages are calculated with the expectation that the customer will provide a tip. The customary tip is 20% for satisfactory service. If you really feel the service was bad you could give less (not the food mind you, but that the server did a poor job). While it is optional in the sense that you won't be arrested for failing to tip, to not tip for satisfactory service is just not right (in the USA, I know this is diff in other countries). But read your bill, as some restaurants do add a tip for large parties.