Is it harmful to your hair if/when you have your hair behind you (resting on your back) when you sit in a desk or a chair?
Only if it gets stuck, or if you sit on it and sweat in excess I guess (I doubt sweat is good for hair anywhere). I don't think that just sitting and having your back against it is harmful.
It's funny you bring this up... lately I've noticed that if I sit in a wooden chair some of my hairs get caught and tug... so I have to lean back and gently pull them out. I usually wear my hair down, so this happens a lot. Also, some of our wooden chairs in our house are duct taped together, so it gets stuck to that too... just gotta be cautious.
-Ardroth