I deliver pizzas, and we do remember who doesn't tip and who does. If I take 3-4 orders on one run, I will on purpose take the persons who doesn't tip last that I know of, and those that I know tip nicely first. And if its just that run, I let it sit until I have to take it sometimes. If your order is late, its not because of the delivery driver most of the times, its because the people inside coudlnt' made it fast enough and/or the shop is backed up. The drivers drive really fast for the most part actually make up time, so dont blame them for it not being exactly 45 minutes or so from the time of order, especially on a friday and saturday night.
So, do yourself a favor, tip at least 2 bux. Like was said earlier, we work for tips for the most part.