FWIW having installed a full 7.4.1 Atmos system including ceiling speakers, I would really recommend not bothering with the ceiling speakers. What I discovered after all the hassle of installing them in the ceiling is that, just like our lives, the vast majority of movies take place on a flat plane.
The amount of times the ceiling speakers are actually used are minimal, and it ends up being more of a gimmick than anything else. In most movies they're not used at all. In others maybe they're used once or twice, and if they weren't there it wouldn't really matter. Only in a rare movie, like a submarine movie, that actually takes place on a 3D plane, are they used to any large effect. Even then if you just had a 5.1 setup you'd get 95% of the effect.
You're better off spending more on solid speakers for a 5.1, or maybe 7.1 if you really insist (7.1 is already a marginal improvement over 5.1), rather than diverting the money to ceiling speakers that are barely used. If you're going to put in ceiling speakers, but in two speakers, don't bother with front and back as in you can barely tell the difference, in addition to the fact both are barely used.