Found this on the net (have WAY too much time on my hands......:
The Case for 'Under'
Evidently bourgeois lavatorial etiquette vis- Ã -vis sanitary stationery demands that a toilet roll is hung 'under', ie with the flap 'wallside', if one does not wish to betray one's lower-class roots. In a working-class home the paper hangs into the room, although which room is not always clear... the dining-room perchance?
And since blue-bloods most certainly would not sully their hands hanging their own toilet rolls, if indeed they'd stoop to wipe their own precious derri è res at all, this assertion would not be inconsistent with the fact that cat-owners, a great many of whom are most assuredly middle-class, adopt the under-hang arrangement 'because cats are inclined to stand on the bog-seat and scratch, as cats do, at the roll. If the paper goes over the top, said bog-roll is soon in a pile on the floor.' Cats, it seems, cannot have the same fun with a back-roller.
Practically though, there are some sound reasons for under-rolling. In the first instance, taking advantage of the friction generated between the wall and the roll, pulling off the next sheet from the back-side should be marginally easier. Moreover, under-hanging is shown to afford the opportunity to take the paper with one hand, when, for example, changing a baby's nappy on the bathroom floor. This manoeuvre is undertaken by grasping the loose end in hand, the back of which is placed firmly against the roll to stop the roll from turning, whence the next sheet of paper can be torn off.
The Case for 'Over'
The key reason cited for over-hangers, ie those who choose to hang the roll with the flap into the room, is hygiene-based. By 'overing', one ends up touching, with the wiping hand, fewer squares of paper that the next person will have to use, which does seem somehow more hygienic, if not entirely nauseating. Reaching behind or under the roll is evidently more likely to bring the wiping hand into contact with the roll. Yuk.
Moreover, consider the tropical rain-forest condensation that can gather on bathroom walls. A back-hung roll's flap tends to adhere to the wall - the major disadvantage of which is that the wetting destroys the integrity of the paper, typically failing catastrophically at the moment of truth.
In the end, however, the bottom line is that the most important thing is that the paper is there when you need it.
Hear hear!