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

@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.