when I delivered I considered $3 to be a good tip, but it all depends on where you live. If you live in LA or NYC I'd say $5 since the cost of living is higher there, but where I used to deliver (Fargo, ND) $3 was a nice tip with $1-2 being average. Just try to keep in mind that most of the people who deliver pizza, or any type of food, use their own car and their hourly wage will not usually cover the general wear and tear on their car. The only reason I was willing to do it was because of tips.
For those that don't tip, I would assume you'd rather pay more for your pizza than leave a tip for the delivery guy.