I always fill the tank.
I agree its hard to measure mpg if you don't fill the tank. As for the rest of it though I think the longer you track mpg, the better you can nail down your average mpg. Right now I'm hitting the high mark at 18 mpg over an average of 3 weeks. As the A/C season kicks in that may come down slightly.
I live in an area where the cost of gas can drop by 20 cents just by crossing the Idaho/Washington border. So its worth it to fill up at the cheaper location regardless of how much gas is currently in the tank.