I've really never understood why this is even a question. I would have thought that 100% of all humans would want the toilet paper to go over the top of the roll much in the same way that none of my fellow human beings want to be stuck in the eye with a hot poker. Fact is that when the paper goes over the top you have easy access... and we're done. That should be the only reason that matters.

If the toilet paper goes behind it often lies right next to the cabinet or the wall. I there is any moisture on the cabinet or wall the paper will be attracted to it and stick. Even if it doesn't you still have to either scrape your nails against the wall to access the paper or pin the paper against the wall and pinch it to be able to pull it off of the roll. Who needs such aggravation?