No its not a silly question as we are asked this alot (we are told this by many builders!) - A Chimney / flue system generally performs better the straighter it is vertically, this keeps the friction of the gasses to the flue liner to a minimum and therefore keeps performace maximised, this is  due to less resisitance compared to a bend in the flue which also then increases turbulance. So in short the straighter the flue the better the performance. Traditionally you would install a set of bends on the flue to move the flue route over for the bedroom fire above, which they would try to centralise.