I'm "thumb in" myself. Never tried thumbless, but I don't think I'd feel too safe. I remember a had a powerlifter spotting me one day when i was trying for a one rep bench max, and I got stuck about half way. He started yelling, "squeeze the bar, tight as you can!", and I finished the rep. Maybe it's psychological for me, but that was a huge piece of advice that I always revert back to if I think that I'm gonna fail the press. But with thumbless, I don't think that you have the same ability to squeeze as hard.