I've only been asked if I've actually filled it when the miles driven was really small and it might have been a freeloading trip.

I generally try to fill it to the first "pop" in the fuel pump at a station within a few miles of the airport, but even different styles of filling the tank may get you different full tank rates more than any other factors.

When I've gotten a tank less than full then I go ahead and return it full for the next person. When I've asked, they've given me a credit that was generally about double what I paid for the fuel. With the changes in price most recently it might not be quite as good of a deal, but I would bet they still give you a larger credit on the bill than you'd pay for the fuel.