There are many variations on how surround speakers are incorporated into Home Theatre installations – most of them are wrong. Admittedly in most cases these are compromises brought about by room layouts, aesthetics etc.
Surround speakers should be situated on the side walls of the room, one to two feet above ear height. If this is not possible they can be located on the walls behind the seating position – in a 7.1 system both options would be used.
There are many speakers designed specifically as surround sound speakers. Whenever possible these designs should be used.