Buying one gallon of gas at a time is typically a poor idea (filling up is typically a poor idea as well). You could easily get stuck some place, but that is not the main concern of the poor. You can get optimal efficiency out of your car by adhereing to several maintance and operating principles.
  • Don't fill the tank: You add extra unnecessary weight when carrying around 12-20 gallons of gas (that times about 8 or more pounds per gallon is a lot of extra weight, compared to half of a tank.)
  • Don't go out of your way to get gas: Usually, driving off of a highway and back on is bad, if you only put a few gallons in at a time. If gas is "on your way" that's fine, but don't keep yourself low on gas because you will gradually spend money on your trips to and from the gas station.
  • Keep your tires well inflated: Keeping your tires well inflated will give you 5% better gas milage, as it reduces the drag on your car. Also, keeping them clean and free of sand and debris can make them grip better, and generally more safe. Letting tires wear down can be dangerous in certain conditions, and destructive over time. Also, get rid of the snow tires as soon as possible. It's better for the road, and your milage.
  • Use your vents instead of the windows: If you are using a "free" cooling mechanism, using vents can keep your car more aerodynamic than having the windows open, and thus you will do about 5% better on gas on the highway.
  • Don't use power windows, locks, sunroof, etc very often: This will make the engine work harder when you are using it, and thus it will suck up more gas.
  • Drive the speed limit: I know it hurts, but the speed limit is the way it is for safety and fuel efficiency purposes. Most cars will do about 20% better on fuel at 55-65 than at 75 or higher. It's a sad reality, but the people who designed the highway know what's best for you, even if you are in a hurry. This is a noticable difference when driving all day, or over a long distance.
  • Read your owner's manual or contact the dealer: There are plenty of ways to get optimal performance out of a car, if you simply listen to the people who built it. They know what is best for their baby, so crack open the manual, or drop a call to a well respected dealer for your type of car.
  • Be smarter about braking: My dad hates to brake; he tries to coast everywhere. Why? Because in gas-powered cars, breaking is lost energy (in hybrids, breaking recharges the battery by recapturing motion). You lose the gas it took you to get to that speed. Driving "stop and go" makes you use a lot more gas than coasting to places.

As you can see you can do much better with very little money by driving smarter, rather than putting one gallon of gas in at a time. People tend to think they drive a little better on vapors, and whether or not it may be true in some cars, it is perhaps a better idea to take your one to five gallons of gas, and spend it more wisely than miserly.