I'm right-handed (and nowhere near ambidextrous), but have always looked through the viewfinder with my left eye, closing my right. I've tried using my right eye, but it just doesn't feel natural. I wish I did use my right eye (and I may have to practice / train myself to do so more), because I think I'd have less oil from my nose / cheek on the back LCD that way (maybe not).
I've been wearing reading glasses for about 10 years, and probably need to get prescription glasses, but have been putting that off (vanity). I don't like wearing glasses when shooting, and I have the diopters on my viewfinders adjusted all the way in one direction, so that the in-focus image in the VF is clear.