Home cinema designers need an artist's eye with an engineer's mindset. It is not enough to simply make guesses and take a purely aesthetic decision when deciding the best location for the speakers that make up a home cinema audio system. Inevitably there will be compromises necessary to fit in with other design elements of a space, but ultimately, the best sounding home cinema is the one where these compromises are best balanced.
We are lucky when designing environments in which to watch movies, because when a movies is created and the soundtrack is mixed, the environment in which this happens is largely reproducible in the home. This is done according to the ITU-R BS.775-2 (International Telecommunications Union) recommendation for 'Multichannel stereophonic sound system with and without accompanying picture'. Ultimately, what the home cinema designer should try to reproduce is the same effect as in the mixing studio.

Speaker positioning
If there is to be only one pair of surround loudspeakers, as in a 5.1-channel system, they should be placed within the angular range +/-100 to +/-120 degrees.

Recommended speaker positioning for a 5.1-channel system.

If there are four surround loudspeakers, as in a 7.1-channel system, the side loudspeakers can be placed symmetrically within the angular range spanning +/-60 degrees to about +/-100 degrees, and the rear loudspeakers within the angular range spanning approximately +/-135 degrees to +/-150 degrees .

Recommended speaker positioning for a 7.1-channel system.

A 6.1-channel system only has a single centre-rear loudspeaker, but it is recommended that the signal be split between two rear loudspeakers. The centre speaker should be placed directly in front of the prime listening position with the left and right speakers placed from 22.5 degrees to 30 degrees from this centre line.
If we place speakers in this way we will achieve a continuous, smooth sound stage where the frontal sound stage is continuous and smooth from the left to the centre to the right speaker, and not biased to one side or the other. The playback system should be capable of providing a sense of envelopment, giving the listener the impression of being immersed in non-directional sounds, such as the reverberation heard in large rooms, halls, and corridors, crowds in stadiums, environmental sounds and so on. Ideally, this should be done without listeners aurally locating the surround loudspeakers, except when sound effects are directed to a channel/loudspeaker for that deliberate effect.
A good test of correct speaker positioning is to lead someone into the room who does not know where the speakers are situated. Sit them down in the prime listening position and ask them to locate your five or seven speakers. If they can do this, you still have some work to do. If they perceive a continuous 360-degree sound stage, then you have been successful.

The speaker placement should give the listener the impression of being immersed in non-directional sounds (image courtesy of FAB AV).

The other key engineering task when positioning speakers is to locate the subwoofers correctly. Because of standing waves, no two seats are likely to experience the same bass response. Equalization can improve the sound quality, but the seat-to-seat differences remain, unless multiple subwoofers are employed in a manner that simplifies the standing-wave patterns in a room. There is a myth that because bass frequencies below 100Hz are omnidirectional, it does not matter where the subwoofer is located, but this could not be further from the truth. The aim is not only powerful, controlled and deep bass, but an even bass response across multiple listening locations. Even the most sophisticated equalization can only go so far in achieving this, so you need to get the physics right in the first place.
Multiple subwoofers should be positioned with care in order to mitigate the effects of standing waves.