If they've added 15%, regardless of what they call it, "service charge," or other, it seems reasonable to give them another 5% to 10%, unless the service was horrendous. People should expect to pay at the very least 15% when they are in a restaurant. If you leave no tip at all, in a place where there is no built-in 15%, and you do so because of poor service, I think that's just as douchey a move as a waiter giving you that poor service to begin with.