Like the guys said above the index finger on the rings if you bench with a regular grip; if you do the reverse grip thing for some reason you can have your pinkies out to the rings I believe. But read the rule book!
You need to find the grip width in which you are the strongest if attempting to lift a max weight. What I did is put a bar in the floor with a 2.5# plate on each end to hold the bar up a little bit. Then I did push ups at various grip widths until it felt the easiest. I typically use that grip on the bar. So what's the difference in doing it that way vs just benching until it fells good? IDK but with the push ups I could feel all the back muscles and all that working together. Maybe something about laying on your back dulls that feeling? Maybe some of the smart m*ther F^cker$ on here can shed some light on that.
Hope this helps.