I did say that I would undertip for poor service, but to me that generally means 15% instead of 20%. Only once have I left less than 15%, and that was when we were completely abandoned by our waiter and, after waiting 45 minutes for the check, had to find a manager to get it.

My sister's been told that you can always spot a girl from New Orleans, because they always overtip. I suspect that's because we're not good at math thanks to our crappy school system, and 20% is easier to figure out. XD