Under places the end close to the wall and other potential sources of contamination when you reach for it. Over is the one true way of the TP. This under crap is perpetuated by interior decorators who think hiding functionality is a virtue.
Over is certainly preferred, but if you have a cat (or a toddler), under is a necessary evil.
This works on toddlers for about a week until they figure out how to unravel it the other direction.
My kitten still unrolled a bunch just by pulling the loose sheet.