I have occasionally filled up a rental 15 or 20 miles from the return (when I did not know the area and was not sure where a safe fill up location was near the airport and google maps was no help) but I do make sure to top up the tank past the first click (usually just a few cents more) to try and be fair, and I am usually driving more efficient vehicles (ie not an Expedition or Suburban). Generally I do it right outside the airport otherwise if I've noticed a station on my way out. I rented about 30 cars last year (RDU, AUS, HOU, IAD, LGA, TPA, GSO, CLT, DCA, YVR) mostly from Budget (couple Avis, Enterprise and Alamo thrown in) and I think they were all full except 1 (maybe 2, but only remember 1). I don't think this is too big a problem, at least in my experience.