I fill up whenever the car/truck is below 1/4 tank and I've checked gasbuddy.com for the cheapest on my route. Unless we're on a long single day drive down the interstate, then I'll let it get to nearer to 1/4 remaining.
Sure you'll get fractionally better mpg around town but I think the benefits of keeping the tank nearly full outweigh the cost. in no particular order: 1. 9/11 ring a bell? 2. less condensation space in the tank. 3. If your really low you might need to get gas in a less than desirable location.
Ed
