For PCs, it's about comfort over long periods. For cinema there are other factors to take into account: Ideally you want the sound to be level with your ears. You also want the action in picture to be level with the sound. I'm not sure why it's generally recommended to have the centre slightly...
Indeed over a mantelpiece is not ideal.
That's not because looking down is better or more comfortable, it's because it'd be shit looking at people feet if they were on a stage above you.
Ideally the centre of your screen should be slightly above your eyes (with the bottom of the screen a bit...