I have never gotten the "mix in mono" bit. Over the years so many people have explained it to me at HR and I concluded long ago {and this is still my current position} that it's pointless {for me, not as a general thing or for those who specifically swear by it} to mix in mono to get my levels if as soon as I pan things, the relationships change. I always remember Massive Master saying something like he wouldn't even consider getting to the stereo part until everything was sitting nicely in mono. I have never understood that but he knows what he's doing and why and therefore it's one of the ways of going. One of the things I always liked about the art and craft of recording and mixing is that there are a variety of ways of getting to the same point and it would get pretty tiresome back in the day when people, be it here, in books or on videos would advocate only one way as being
the way ~ and it would usually be their way.
Mind you, I've found that in life, to be honest, not just music.
When I mix, there may be songs that involve instruments or vocals moving from one part of the stereo spectrum to the other or other things that keep the relationships of all the elements fluid in one way or another.
Having said all that though, I may well have parts of a song that turn out to be mono in that everything at that moment gravitate to the centre. And something that all the "mix/check in mono" discussions definitely did for me was to breed a certain curiosity. So whenever I do a mix, I'll listen to it on a variety of systems, a couple of stereos in different rooms, a small boom box, the computer speakers, the car, the ipod and through headphones, one of which has a mono setting. Over the last 14 months I've done about 87 mixes and only on a couple did they sound a bit ropey in parts when I listened to them in mono.
I actually do really like mono recordings from the 50s and 60s though. During the summer, I converted many songs from my Beatle collection {well, up until "Abbey Road" & "Let it be} to mono as I found they just sound better than the stereo ones. Same with the Pretty Things' "SF Sorrow." That was quite a turnaround for me. But for my own stuff, I am and always have been a stereohound.