fuel tanks

i just tried flying the king air 350 with fuel in only the 2 aux tanks. 100% in each one. the two main tanks were left empty. i was checking to see how it would fly like that. all i can say is WOW!!! it's like a brand new flight model or air file. haven't tried this with the two main tanks full and the aux empty or the main tanks at less than full and the aux empty. my question is how do you know which tanks to fill and which ones not to fill and why? it obviously makes a huge difference. i'm aware of using less than full tanks but not as to which tanks should carry fuel first and which can be let empty. this is very important because it makes a BIG difference in how the aircraft flies and not knowing any better it's easy to say there's something wrong with the flight models. also not knowing any better i would have tried just filling the two main tanks and leave the aux alone which i'm about to try. to fill or not to fill, that is the question. william