I rent regularly from Avis and they are fairly upfront in the rental office with signs and sometimes verbally mentioning the $13.99 charge if you drive under 75 miles and don't show a fuel reciept. Luckily I am usually dropping off at the airport and the agent with the mobile printer checks me back in and I can ensure no fuel charge.
But with the price of gas at $4+ if the car does 16-20mpg then I reckon if you have done 60-75 miles then it is less hassle to just pay the $13.99 and you might even be a few cents/$$ better off!