I do if the delivery is on time and the food is at least warm. If they take over an hour to get to my door, and the pizza is stone cold then they get nothing, I used to tip all the time no matter what, but after a while I was just getting fed up of the food never getting here quickly and always being cold - to the point that it was inedible - so I started cutting the tip down or stopping the tip all together, finally I just stopped ordering from them.