Ultimately you can place your speakers wherever you like, or you can follow the advice, published by the organisation who invented the standards, explaining how and why you should follow their advice to obtain the overall sound-stage that they intended for you to hear, to achieve maximum aural immersion.
The advice posted here not only follows the advice from Dolby Labs, but is also based on considerable personal experience from the posters.
Speakers not being 'a million miles away' from where they should be, often means in audio terms, they may as well be a million miles away. Atmos at the end of the day is a height layer designed to be fired downwards or reflected downwards from the ceiling, to achieve the desired effect. You can choose to wall mount your speakers and they may provide some form of sound-staging, but they will not be providing the effect intended, by Dolby Laboratories, or the film's producers to achieve the desired atmos effect.