I think for best effect
The top speakers need to be installed as Dolby Atmos suggestion
Just like a pair or two pairs of speakers directed towards your head like your front speakers to create a sound stage
For DTS X and Auro, because they use more speakers, they can use high speakers instead
Otherwise only a single sweet point can experience the Atmos speakers effect
You can verify yourself by moving your listening position
The effect is only prominent in one sweet point
A bit forward or backward, it sounds totally different