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

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