How bad is it to place 5.1 rears from behind rather than on the sides?

5.1 rears are just that. Avoid the sides because that was not the mixers intention.

There have been many opinions on this as well as the dipole vs bipole type of sound.  Technically speaking, the 5.1 surrounds are meant to be placed on the left and right walls directly inline where are you sitting.  That is how the soundtrack is mixed.

There are the 7.1 "rear surrounds" that are placed on the back wall, but that's an additional channel of processing.

That being said, it's a decision/compromise you will have to make.  I would not say that it's necessarily "bad" to place your 5.1 surrounds on the rear wall.  It still give you the surround effect, just maybe not quite as accurate as the movie production mixers intended.

How bad is it? ..... https://www.youtube.com/watch?v=o5pZoxH1lSI

@auxinput You raised a very good point. After your comment I now watched a couple of YouTube videos and became aware of the dipole idea. Looks like dipoles on the sides can actually be good for two reasons: the ears of those setting on one of the sides of the sofa will be in its “null” area. Second, the sounds effect will be more immersive. What do you think?

Also, I realized that my rear in-walls pose a problem on their own so I’ll return them anyway.

My friend had a setup where his surrounds were in-ceiling speakers behind the listening position and it worked really well, so that might be another placement option if the ceiling is workable.

I have my sorrows sitting on the floor pointing up at the ceiling behind the sofa. I had to raise the volume some but during the movie you can’t really tell where the sound is coming from anyways. I think it actually has a better effect because the sound comes from behind the furniture I’m kind of spooks you

My experience is that for 5.1, the sides are the right place for them.  If you are having trouble deciding, find a cheap pair and try them out, I think it will convince you. :)

behind firing up works for me.

small 5.1 home theater, 13' ears to front speakers (see my system posted here if you like)

my narrow rear surround speakers are on the floor, on their backs, and fire up, using the 6" space between the back of the sofa and the wall kind of like a horn or transmission line. 

rear surrounds, like a sub, should not be apparent generally until you turn them off, then the sound field collapses to the front. certainly apparent for special effects, flying heliocopters, ... but many people have them too loud. use your receiver's software, and avoid over-doing it. in the wall behind me would not be good, and I think the small drivers get a boost by the air channel on the way up.

The dipole surrounds have the woofers/tweeters wired in opposite phase of eachother, so yes it does create kind of a NULL where the couch is because the opposing drivers are cancelling out the sound (if the speakers are mounted directly to the sides).

At this point, it becomes more of a personal preference.  Dipole speakers are meant to produce a type of sound that is more ambient/spatious so that you cannot tell where the sound is coming from.

In comparison, Bipole or direct speakers do not do this.  I had researched this a long time ago and decided that I wanted direct sound because it is more solid and I can hear distinct sounds coming directionally from either the left surround or right surround.  It gives a bigger impact, but it is definitely a "direct sound".  You might feel different on what kind of surround you want.

I should point out, coming from a short stint in actual motion picture auditoriums, I always felt the dipole satellites were a scam. I’ve been very happy with normal speakers as surrounds, especially with the addition of 2 rears.

I have a feeling though that some of this has to do with a desire to do more with less. A typical surround auditorium usually had 2 speakers per side, and sometimes in the rear depending on the size of the room. There’s also some disagreement by the recording engineers about what they want surround sound to DO. Is it strictly spectacle? Is it immersive? Should I locate a bullet that just wizzed by or should the sound be more diffused?

Fortunately audio keeps progressing into giving the movie makers more control over these choices, and they are less dependent on built in obstacles in the surround technology itself.

Thank you all for sharing your valuable experience. It sounds to me that weighing out the different circumstances of my house, monopole sides will work best for me. Though before mounting them on the wall, I’ll first give them a try both on the sides as well as on the back.  

There are some surround speakers which have two pairs or drivers and are switchable between a dipole or bipole configuration.  If you really wanted to see for yourself, you could try the "dipole" sound and compare with the "bipole" (which essentially give you the same effect as monopole).

You are so close to a nice 7.1 set up with all 7 speakers on stands. All you need do is pull the couch forward and little, enough for speakers to be as they are in in your first figure, behind you. a bit wider than your L/R pair. 5.1 to 7.1 is addtional speakers on the sides, toward the back of the room maybe 3/4 of the way. You could then add 2 or 4 speakers overheard in the ceiling and you’d have ATMOS! Small rooms actually are better for this if you dont over power them with big speakers. Nice small speakers like Genelecs or soemthing like that would be ideal.

Erik is right, dipoles are a thing being "sold" to consumers; nothing is mixed that way or built that way in the studio. I would not use them as nothing would sound its best that way. Especially when standard speakers work fine and are easiy to move around until you get a good "blend" between front, sides and rears (where you dont hear a big dip in level between speakers from a source that moves across all of them).
Brad