I voted for Arch, but I'm not so sure about my ass actually coming off the bench, I like to left my forehead to the bar then slowly drop my shoulders and neck flat on the bench while bring my ass up a little to prepare myself for reps, but that puts the arch in my back. My ass and feet stay planted the whole time. After tearing my anterior deltoid, I started watching videos of professional trainers teaching bench form, and that seems to be the overall consensus for benching. I've also slowed my reps a bit and focused on keeping my elbows at about 45 degrees from my lats.