Quizzo - couple of good reasons to fill your tank rather than keep topping up.
If you get find fuel at a good price, it makes sense to fill up.
Having a fuller tank gives you more options on where you fill up, this can make the difference between getting robbed at a motorway service station or being able to comfortably get to a cheapie.
Air space in your fuel tank isn't ideal, more air means more potential for condensation.
The difference between a half a tank and a full one is slightly less than 60lb, which really ain't going to dent your fuel economy. Having your tyres under-inflated by a couple of PSI would do much worse long term.