I analysed the music "That's what I like" from Bruno Mars, mixed by the maestro Serban Ghenea, and I fund interesting things :
Next to the fact that in full mono the song sounds very good, I noticed some of the instruments are not EQd on the low end of the side part. Here is a screen shot.
My ears detected 2 instruments, the lead synth and the lead bass of the music.
On the Mids I detected the same lead synth, bass, and now the lead voice and a piano which gives the rhythm of the music.
On the High Mids it's the same things like before with the lead percussions + hit
On the Highs it's all the same things I mentioned before
So, to resume I think it's important to choose what we would like to keep for the sides and maybe do it during the mixing process, unless we have a lot of stems during the mastering process. I think I would prefer do that during the mixing part. It's possible I missed some instruments, but the main infos are here.
Now let's speak about mixing in mono : I noticed the mix sounds very good in mono, the sides are only there to sublimate the song. There is a possibility the mix has been mixed in mono, but of course I'm not sure. It could be an idea to put last on the Mix Buss Ozone (or an another plugin) a click on the mono button, then mix and during the process try to bypass the mono to see the result.
I (and the world) like the work of Serban, I think it's not an accident he receives a significant part of the next hits to mix. As there is no material about his mixing technics, it's the only way to find out some technics, and maybe integrate them into our own mixes. I think it's a sure value. What do you think ?