There have been a few times I've done 30 or 60 day trials. Within those times when I was a legit member I have walked past busy desks without being questioned (MIA, SLC) as well as gone in through side elevators (JFK) without intention of "sneaking in" but bypassing the check-in desk. Once when I needed to speak to a competent res agent I asked someone to guest me in and they did. It's not too hard to do, but a blitz past the desk isn't the way to do it.

That said, I did "self-upgrade" at Thrifty in MIA once... but only because a Sebring Convertible was parked in the space number I was assigned, instead of the Sebring Sedan I was booked in. Not that much of an upgrade to be honest...