If you’re primarily a music listener, place them on the back wall of your listening/viewing room but a bit closer to the side walls than the “Rule of Thirds” indicates. Make sure the tweeters are at approximately the same height as are the tweeters of your front Left, Center, and Right speakers
>>> Diagram of surround sound speakers for music listening
If you’re a movie mogul (or just enjoy ‘em), mount the surround speakers on the side walls flanking your primary listening/viewing position. Make sure they’re slightly behind your couch or chair and at least 1.5 to 2’ above ear level.
>>> Diagram for surround sound speaker placement for movies 5.1 system (without subwoofer)