I'm just providing the Dolby Atmos guidelines. I would think they came up with them for a reason.
Most home theaters are smaller than the Central Listening Area (CLA) of a commercial theater. The Dolby Atmos guidelines for a commercial theater also call for the overhead speakers of the CLA to be aimed straight down.
One can experiment themselves with any of their Atmos speakers (or JTR 8" coaxials). Setup them up as mains and compare from pointing straight out to having toe in. Sit at the same distance as the Atmos speakers will be overhead. Connect them to the Atmos channels and play an Atmos demo or something with panning. If the soundstage gets wider with them pointing straight out, then you will get a wider panning area for Atmos with the speakers pointed straight down. You aren't trying to get a super strong stereo image between the speakers.
I just tested with the Dolby Surround 7.1 "Sphere" Trailer and prefer coaxials pointed straight out for the best effect.
https://youtu.be/t4G9wSjNyjk?t=7s