Something else I've wondered...if you get spectacularly bad service (the kind that makes you leave comments on the company web site) should you then be able to deduct from the bill?
I've always been told that if the service is really bad, you should leave pocket change -- just a few coins. That action conveys your message better than no tip.
What I really hate is that when paying for a meal with a big bill, the waitress returns with only big bills. Then you have to ask them to break a $20 in order to leave a tip. That is always done on purpose, and I'm certainly not leaving $20 for a $10 meal.