I do planing on going with 4 speakers Atmos and I Have 8' ceilings. Stupid original builders of this home did not spring for the 9' basement upgrade.
If you have the option of going in-ceiling, then that will give you a little more separation than going on-wall like I will have to for my Atmos when I get to it. As Bill mentioned, size of room matters too, and where your seating is in relation to the speakers.
The Dolby guidelines are very helpful for understanding different placement options... How to apply them is the trick, perhaps the art, of building a good rig.
My rears are wall mounted above a closet... tweeter is 82.5" H on-center. I've considered turning those into Rear Heights if I can find the right processor that will allow me to do Front Height and Top Middle for a 5.x.6 arrangement. I have also considered the possibility of a Rear Center as I do have space in my room to put a single speaker on a stand at the rear... which could give me a 6.x.6 arrangement.
Again, this is all based of figuring out what works for your situation, and having the right gear in place to manage it.
My current processing capabilities will only allow for up to 7.x.4, with the bed layer allowing for a Rear Center... but Front/Back Height speakers are less desirable that Top speakers. In my room, Top Middle with Front/Rear Heights would give me a very immersive Atmos layer over a very solid 5.x Bed layer...
Love me some 1st World, 21st Century problems!