Average 25-30%, minimum 20% for a tip in D.C.? My guess is that the person who came up with that standard works as a waiter or waitress in D.C.

15% standard, 20% very good, perhaps a bit over 20% for superior service. I live in D.C. and dine here a lot, and have never gotten any kind of an argument over that approach.

However, just to get my two cents in on the overall topic: I find it interesting that we (including me) allow ourselves to get tremendously agitated when some poor waiter or cab driver at the sub-minimum-wage level tries to squeeze an extra dollar or two out of us. But when slick, well-educated, wealthy lawyers and politicians casually scoop hundreds and thousands of dollars out of our wallets, we barely wimper, and sometimes even proudly support their right to do so.