We occasionally hire a car whilst abroad and learnt our lesson from Kevin when he had trouble returning a car in Mexico. He had declined their insurance as he was covered on his credit card. When collecting the car the rental agent went around and ticked on a sheet the areas with damage (pretty much all the car!).
On return the agent tried to say there was further damage to the rear bumper, this way he would be able to make a claim through Kevins credit card insurance and get all the bumper damage fixed. Kevin had a hard time standing his ground to get them to agree it wasn't his damage but from this we have learnt a lesson.
TIP - When hiring a car not only go round with the rental person ticking the damaged parts on the rental agreement but also be seen to photograph them as well and if possible get the agent in one of the shots. This way if there are problems on return you have proof of how the car started out but having seen you do this they are less likely to try it on.