I rented a car from National from 12/28 to 1/5. I pre-paid for gas because I knew we were going to do a lot of driving. I ended up paying a price that was lower (by 2 or 3 cents) than any gas station we saw during our visit.
The gas light came on as we were leaving OKW and we made it back to the airport without filling up. Did I leave a gallon or so in the tank that I paid for? Probably, but I didn't have to worry about refilling the tank at a gas station close to the airport.
If I wasn't planning on driving much, I wouldn't have paid in advance. But if I'm doing a lot of driving, I don't have any problem paying for the gas up front.