I do as sinnedk, for the most part: double the tax. It's fast, easy, and about 18%. If service was really good, I'll add few bucks. If service sucked, I'll tip the tax (that pretty much never happens).
Anyway... I am glad we had this discussion. It's interesting to see other people's points of view. Whether we all agree or not is irrelevant - always a good thing to discuss good topics. Thank you to everyone who participated.