Yes you can install a headset or BB without first prepping the frame. What happens is you will be pressing the cups of the headset into the head tube and if the upper and lower surfaces aren't parallel then the bearings wont be perpendicular to the steerer tube. This misalignment will be slight and basically it will not cause too many issues...bearings will wear out faster and maybe the fork is very slightly harder to turn (but there is so much force you will easily overcome this). Shouldn't cause major issues. The BB is the same deal...if you don't face it the surfaces wont be perfectly parallel and you will wear out your BB faster....this was a big issue with cup and ball BBs and is also with outboard BBs but not so much with cartridge bearing BBs like most will use.