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


I’m building a new setup and planned placing my 5.1 rears in-wall directly on the sides and perhaps pushing the sofa forward a bit while watching a movie. I especially wanted inwalls to avoid my youngsters messing with them and bought the Morel xbw600 two days ago for that. But renovating now, I see the only option to make them in-wall is behind me in 90 degrees (separated up to 3 meters apart).

So I’m facing a dilemma: do I place them behind or do I return them and get satellites instead that I’ll place on the sides (1.5 meters from me in each direction)? What do you smart people advise?

Illustration: https://ibb.co/86wc61J
thenoob1
5.1 rears are just that. Avoid the sides because that was not the mixers intention.
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.
@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.