Me Da reckoned that there'd be less stress on the holder fixings if you hung it down the back, but I've never bothered to work out the parallelogram of forces thingie. Anyway, ours hangs down the wall in loving memory.
Anecdottle: when our daughter's partner has been here we always find the bog roll hanging down the front, as if he had a pathological urge to hang it "properly" and was "correcting" ours. Then a while back I realized that he wasn't: he was just using so much that he finished whatever was on the roll every visit and put another one in.
The Inlaw Paw, having been in the RN, often quoted the maxim "one up, one down and a polisher". Modern youth is obviously more prodigal: I read some years ago that according to Kimberly-Clark's statistics, the average American uses 47 sheets a day.