Just checked online...... it states that you only have to show the receipt if requested
"You may avoid a fuel service charge if you return the car with the fuel tank as full as when you received it and, if requested by us, present a receipt for your fuel purchase. If you put fuel into the car, you must use the correct fuel (having the grade of gasoline stated on the car fuel information decal, or on-road diesel). Do not use ethanol fuel even if the car states that it is a flex-fuel vehicle."
Either way, I will book with Enterprise companies. I don't seem to experience the game-playing with them.