A big congrats on that system. I think you got a 600v4 as well, correct?
I have zero experience with Atmos, in-ceiling and in-wall. The best advice I can give you is to mock it all up and listen to it somehow before drilling holes. I think Blair's advice makes sense for all speakers except maybe the front left and right. I've found with my M5s, tweeters at a few inches below ear height helps them disappear better without any loss of treble detail or air. At that height, images in well-recorded music appear behind the M5 (as they should) rather than at the M5. I've found that to be dependent on MLP distance. At 10' away in my living room, tweeters at ear height works but at 13' away in my main room, tweeters below works better. This may also be dependent on the different adjacent reflecting surfaces in each room.
Since you have in-walls, the above may not apply. I am really curious if your in-wall M5s will give you the same holographic soundstage I enjoy so much.
