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

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.