I'm 6'5" myself and my back would get sore after a half day or so. But, having tried loupes the last few days, they REALLY save your back. So much so, in fact, that I felt great while using them, and no pains at all. I don't think Dentistry in general is any worse for you if you're tall. I think it's just one of those things where the world isn't really built around really tall or short people so the extreme's suffer regardless, whether you're a Dentist or not.
I would definitely recommend loupes. Being in the sim clinic in one thing, and being in the patient clinic is another. At least it is at our school. Our sim clinic sucks and the tables aren't adjustable. Only the chairs are and at their lowest, my back would still hurt. But with loupes, it really is night and day. In fact, I have to now raise the chair which helps a lot. Get them 🙂 You'll be happy you did.
Cheers