I have a very similar condition, which my optometrist calls "right-eye heavy", where landscapes are always tilted down on the right-hand side. She says it comes from the way your eyes process scenes, when both eyes are open, you naturally compensate but since you close one eye to focus... She also said only photographers complain about it.

Bubble levels and grids are the only things that work for me, since I know it is not level but I don't see it.