Disneysteve has hit the nail on the head. Don't ever prepay for gas in a rental car unless you can guarantee that you will be able to coast into the car rental return lot on nothing but the fumes in the empty gas tank.
On our trip to WDW in January we drove to WDW from the airport, used the car to get around the property to restaurants, etc., used the car to go to Clearwater Beach for a day, got lost in the Tampa/Clearwater/Sarasota area for a little while and returned to the airport on the last day -- all without using a full tank of gas.
The best solution is as cra-z-4-disney has suggested -- fill up at WDW and use the full tank of gas to get you to the airport and also get credit for that full tank of gas when you return the rental. 'Tis the best of both worlds.