It really depends on what type of driving you intend to cover, whilst filling up.
Casting your minds back to your schooldays - you'll recall talk about inertia, momentum, etc. in essence, the more mass there is to accelerate (and decelerate as well, remember), then the greater the force which is required to change velocity.
However, if the vehicle (or object) is accelerated to a constant velocity (on a level plane), then the force which is applied to maintain that speed is equal to the friction the object is experiencing (tyre & air resistance).
Quite a long-winded way to say that on the motorway - filling to the max is best (you also save on a wasteful stop, and paying higher prices), whereas for town driving, keeping it between a 1/4 and 1/2 is probably best.
Science lesson over for today - hope you were taking notes!