Over. I despise under. I feel like it's harder to grab the TP. The only reason I would tolerate under in my own house is if I had a cat or a young child who liked to roll the toilet paper off the roll onto the floor. But I don't.
I don't think guests should be changing stuff in your house. That would be annoying.
In the previous place I worked, sometimes people would put the toilet paper on under when a new roll had to be put on. I hate under and would change it to be over. Luckily, no one would change it back so there wasn't a TP positioning war. I felt like that was OK though because it wasn't someone else's house. I deal with it when I'm visiting someone.