Go to a reputable company and make sure you've thoroughly gone over the 'checking for scratches before you take the car away' bit.
I hired a Fiat near Lake Como a few years ago from a place near my hotel, and when I took it back they kept insisting on there being a 'scratch' and pointing to a perfectly unmarked part of the car. I ended up calling the manager of the hotel to come and look at the 'scratch', he couldn't see anything either, because there wasn't a mark in the first place, of course. It was only then that the thieving rogue gave up and took the car away.
I'd paid, if I remember accurately, a 200 deposit, and that's what they were after keeping. Be wary. [:T]