I usually overtip IMO. Not just servers but the person who cuts my hair etc. I think it comes from when I was a paper boy; just a $1 extra a lot of people gave me when I came to collect made a huge difference. I don't think I've ever left nothing. Of course, it varies on the service. If the service isn't perfect because the place is packed I can understand that. I love when my drink doesn't run out and I don't have to wait a long time for the bill.
Check out
www.stainedapron.com. It is a interesting, often funny, and sometimes gross sight from the server's point of view.