There is no one right or wrong answer regarding side wall treatment. In the context of this post I am assuming you are talking about treatment for the screen channels, not surrounds.
In my book the right way to do it is to measure the speaker's off axis response. You do this using an acoustic measurement package and moving the speaker in say 22.5 or 30 degree increments until you get to 90 degrees. You need to keep the speaker to baffle distance consistent.
The results should look something like this...the response below is for a very nicely designed speaker using a constant directivity waveguide. Let's call it speaker A. The main characteristic here is that the off axis response at angles past 45 degrees is noticeably lower than the on-axis response yet it does resemble is spectrally until 3k or so before a smooth taper down in the highest frequencies.
Speaker B, a normal cone/dome type speaker, is very different. The off and on axis response are very similar meaning the speaker 'broadcasts' much more sound out off axis than the speaker above. There is also an obvious discontinuity between 2-3k which is a typical suckout that appears off axis in cone/dome speakers. What happens is that at the crossover point the directivity of the cone and dome do not match. The cone is beaming and the dome has very wide dispersion. On axis the responses might look the same but off axis they do not.
Why is the off axis response important? Well what you hear in most normal sized domestic listening rooms is a combination of the direct and reflected sounds. Our ears are particularly sensitive to lateral reflections so what you do with them can make a big difference in the sound quality. Suffice to say there is no one right or wrong answer and acoustic designers or companies that always specify the same treatment for side walls are not looking at things in enough sophistication.
The factors that should go into a decision about side wall treatment are numerous and include:
- speaker off axis response
- angle of incidence for the major side wall reflection points, which tells you what the spectral balance of the sound striking the wall looks like
- path length difference for the reflections, which tells you what level they will be at relative to the direct sound, since the further sound has to travel the lower in amplitude it will be
- room use - is it for movie watching or music. For movies the ventriloquism effect is very powerful and so imaging specificity is for me less important than for music
- personal preference - does the client like a very spacious soundstage, pinpoint imaging or perhaps a balance of the two. This is in turn typically related to their content preferences; for example rooms I have designed for people who only listen to symphonic classical you can really design to make the soundstage huge and not worry about pinpoint imaging
Hope that gives you a starting point!