Just from doing it I would say that it gives you a bit more lee-way (with the safeties at least) if you bench press with your back arched. A tip I once got from a fellow lifter in the gym was that lifting with your back arched and with the safeties high enough to match your back arch will give you a bit of a cushion in case you fail the lift, whereas if you do so without an arch you either are going to git the safeties or get pinned.