I try to fill up when it's cheap and pinch when it's not. Usually just 20-30 dollars though and thats because I use 20-30 dollars a day!
2.79 is cheapest in town.
I generally try not to fill my tank all the way. There seems to be a leak or something somewhere near the top of the tank. When it is full, I notice the distinct smell of gasoline (especially when parking on a slope). This is also the reason I no longer smoke in my car.