TV Mounted above fireplace mantel - neck strain?

I have found, directly below the monitor works best for center.

Yes, but if possible, tilt it towards the listeners.  This can improve intelligibility by reducing floor bounce and putting the listener in line with the tweets.  If your main speakers are on axis but center isn't, the center will sound dull by comparison.

Stand up, relax, and notice where your eyes normally look. You’ll notice that our resting eye position has a field of view approximately horizontal with our forehead, downwards. Approximately.  If you say you are looking straight, it's not actually your resting point.  Your mind is just conscious of it and forcing it ahead.  Relax  your eye muscles and your view will shift down. 

That is, our eyes are naturally looking below horizontal. This is why ergonomic guides suggest the top of your PC monitor be at forehead level, and why people who raise up their monitors have an S shaped neck. They hunch over and raise their heads. Bad posture!!

So, if you plan on watching in a recliner, up is good. Otherwise, having the top of your TV about the level of the top of your head is best.