Going back to the original question real quick: Yes, today's RVSM airspace does mandate A/P use from 290-410. You don't have that restriction above FL410, but flying that high w no A/P is hard work for most airframes.

Back during DESERT STORM I flew a C-141 back from Germany w/no auto-pilot @ FL370. It was my CP's leg, and once we reached level-off, he could not maintain altitude and would get himself into a gradually increasing pitch oscillations +/- 500' by over-correcting. After a couple of more tries on his part, I had to take the jet. I got it trimmed out, but it took more work than it would have flying down at 10K.

My lesson learned: Yea, I was able to hand fly a heavy jet up high, but I should've asked for more fuel so I could go a bit lower to help make the jet a bit easier to handle.