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.

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.

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).