I always tip 20%+ unless they include the gratuity, in which case, I figure that they have figured that they can live with their 18% across the board. I dislike the practice, but reading some of the comments here, I understand it. I've never worked in the food industry, but I have a lot of friends who have, and it seems like the biggest dickheads either tip low, or tip ostentatiously high. At least in the second case, you can pretend that the person isn't a dickhead for an hour or so.
FWIW, I've always gotten to know the cooks, waitstaff, and often the owner in any place I've frequented on a regular basis, and apparently, I'm fairly affable, because I would/I do get comped drinks, deserts, new appetizers, etc... on a fairly regular basis. It seems that tipping reasonably well and being polite actually do count for something.