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

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.