Bone of contention in our house. I just cannot grasp* why anyone would want it under. Just makes it ever so slightly annoying to grasp, for zero benefit.
And Andrex have made it quite clear which way round is the right way. When it is over, the puppies are the right way up. When it is under, the puppies are upside down. They are the ones who are right, because they make the stuff.
When the toilet paper is under not over...