I only do the Fuel Purchase Option (pre-pay for the tank) if I know I am going to be driving so much I'll either empty the tank or get close to it, or I have no clue where a fuel stop is at the aiport and I will be cutting it somewhat close getting back and don't have time to hunt.

With what they charge to fill the tank, if you end up fueling it some ways away from the airport, the charges to bring it back up to Full might be more then just FPOing in the first place and bringing her back with a quarter-tank.