The center of the TV should align with the height of your eyes when seated. For most people, that's 48"-60". So a good compromise height is 54" above the floor for the *center* of the TV. Most people mount them WAY too high.

Yes! If you want to get a sore neck trying to watch it, mount it above the fireplace or up high. If you actually want to enjoy watching the TV place it down lower than you'd expect. I just had mine mounted (I'm still moving in) and was mildly shocked at how low the recommended height was. However, it is actually just perfect when I sit there to watch the news while I drink my tea in the morning. Forgive the moving in mess in these photos...