If you fuel up before returning the car, make sure that you keep the receipt from the gas pump.
We rented a car in Myrtle Beach last month and when we filled it up, the fuel gauge didn't go all the way to F, just a couple clicks below. Since we only went about 5 miles on the tank, there was something wrong with the gauge. However, they tried to charge us for 3 gallons of gas.
It was only $15, but it was the principle of it. The receipt had the time printed on it, and the CSR was able to take the fuel cost off the total for the car.
I've rented from every place (I usually reserve through Hotwire) and have really not had any sort of bad experiences with any of these. I just make sure that I mark any dings on the car when I pick it up, so that they don't get blamed on me.