I tip 20% at least 80% of the time. 15% if service was bad. I suppose 10% if the service was absolutely dog****.
When I was a waiter in 2006 I made $3.13 a hour while minimum wage in CO was well over double that. If you aren't tipping your waiter at least 15%, they are paying for part of your meal.
So don't be a dick, tip your waiters.