I have NEVER had any issue with a US car rental agency. I keep my paperwork, know the terms of my rental and have never had any desk agent try anything funny. I have no idea what RM is talking about.
First thing I do when leaving the airport car rental area is look for the gas station. Who wouldn't?
I get miles on EVERY rental that allows DL miles, card is not required for earning (use for hotel cards, too)--which only works IF you have a Skymiles account.
I rent my cars prior to arriving and already know what to include/exclude. I always check for any issues on the body and mark questionable ones before departing. I will turn down upgrades to larger, uncomfortable cars ($ gas and crappy ride) unless no alternative.
I have no idea what "schemes" RM mentions. If the price is right and the terms are good for you, don't fear the rental. I do NOT like third party sights vs 25% off emp discounts.
Cheers!