We covered this a little while ago on a different thread. You can have a seach for it in this forum.
Basically there are the two schools. One never lets it get les than a quater tank so it doesnt suck up the crap.
the one im part of never has more than a quater as the car is so much quicker. Lets face it our cars are new so theres not going to be much rubbish from the tank its self to harm the engin at this stage. Also by always having a full tank i would have though u are actually helping a nice thinck sludge form at the bottom of the tank which could harm the engine rather than making it eat tiny doeses all the time which wouldnt be so noticable.
Plus i am goign to start changing my fuel filter every 6000 miles to be on the safe side.
Less fuel, less weight, more accceleration, better handling (not that i could notice that cos im not a great driver) and most important obviously, since its lighter ur being more environmentally friendly.
Take ur pick of options
mav