Personally, I think that one should pull his or her shoulders together when benching, but not exaggerated. That is, just keep your shoulders back and against the bench and your chest out. Don't let the shoulders come up, it actually destabalizes your rotator cuff and can lead to injury, since the bench is generally a heavy lift. I knew this ex college football player who didn't go pro because of a shoulder injury. He was an offensive lineman, and so you could imagine he was hitting some big weights. HIs shoulder injury was attributable, so he says, to doing a lot of lifts that were hard on the rotator cuffs without, (1), developing the smaller muscles around the rotator cuff that secure it in it's socket, (2), doing lifts that were stressful on the rotator cuffs such as behind the neck pulldowns, and (3), not keeping his shoulders set, (back) when doing lifts such as the bench press. You can isolate your chest very well and get a good range of motion with your shoulders back and your chest out in a powerlifting type benchpress without the unnecessary risk of a shoulder injury. I'm banking quite a bit on his experience and the experience of his trainers and physical therapists, as well as on my own personal experience with hitting the bench and experiencing different ranges of motion. I know that he always stresses keeping the shoulder "locked in" while doing many exercises. When hitting lighter weights, you can develop the muscles of the rotator cuff and in these often you will want to take it out of the locked position and extend it forward, pushing the actual shoulder forward with your arms, as with box step ups with the arms locked.
A little off the subject, he was telling me that it was pretty much required that they had to run 40's around 4.7, 4.8 or better as an offensive lineman. He played for ASU several years ago when they went to the Rose Bowl. Can you imagine a guy near 300 lbs who can push a 450lb or better bench running a 4.7 40? let alone hitting you at that speed and weight? I'm strong, but at 205 lbs, my ass is getting run over.