I made an interesting discovery with rentals in the USA, the cars go from the factory to the rental agency without going to a dealer for pre-delivery. So all the little faults that are normally picked up by a dealer are left in the car for some lucky punter to put up with. I had a Mitsubishi that wanted to change lanes all the time due to a poor alignment and the auto would not change back to first when you stopped at lights.
Walk around the car and check for damage before you drive off. If you can take photos of the car from all angles so that they cannot claim you damaged the car.
Some companies have a little damage sticker hidden in the boot, it is a diagram of a car and they mark the spots that they know are damaged on your car. If you dent your car find the sticker and add the dent to the ones on the sticker.
