I personally find it hard to believe there are so many experienced engineers her who still need to mix in mono. With the exception of certain particularly tricky potential phase contention situations, most of the things that one can check for in mono one should be able to hear without checking on mono after just a little experience. And even those phase issues can usually be ID'd by simply muting/un-muting the potential offending track and listening o it's effect on the stereo mix without having to collapse the whole thing to mono.
For those with less-experienced ears I'd consider it practically mandatory to check a mix in mono regularly during the process. I'm kind of on the fence about mixing in mono, because I can see that as inadvertently crippling what one eventually decides to do with the soundstage, but if they are not going to do anything with the soundstage anyway other than the generic hard rock LCR scheme, then I guess it doesn't much matter.
But IMHO there does come a time - hopefully sooner than later - when one should start weaning themselves off of mono checking as anything other than a final belt-and-suspenders check to make sure you didn't miss something, and start mentally listening for issues in your head without needing to actually do an A/B check every time you add an instrument.
What difference does it make other than bragging rights?
Otherwise mono mixing/checking can kind of become a crutch for the ear, and one winds up with a "fix in the mix" kind of attitude, except in this case it's "fix in the mono". And that can become extra work versus nipping it in the bud. At D-Vision I used to work with a guy who seemed like he was deaf until he switched to mono. Here is a conversation that happened more than once:
"Um, Dave, that rhythm guitar is phasing."
"Sounds fine to me. Besides, if it's phasing, we'll hear it in mono and fix it then."
"You mean you can't hear it now? Or the fact that the keyboard is going to mask the pre-delay verb effect from the lead git?"
And he'd move on and when he got to the mono check it sounded lousy; sure enough the rhy gits were stomping all over each other and the all-important stereo delay on the lead git almost disappeared under the keyboards. The problem with waiting on it is that fixing it at that point led to some actual mix decision changes, causing him to have to undo half of the mix work he had done. If he actually paid attention to what he was doing, and listened closely to the stereo mix, OTOH, he'd have fairly easily known he was painting himself into a corner and could have made the changes early on and not painted himself into that mix corner, saving himself extra work down the line.
Use mono checking, absolutely. It's a great tool. But as you use it, try to teach yourself how not to need it as much and don't let it become an automatic crutch, IMHO. It can actually limit your eventual soundstage choices if you mix only on mono, IMHO, and give you extra work down the line if you can't hear problems without it.
I look at mono checking in that way kind of like I look at mastering. You'll get the most out of it by working as though it's not an available option.
Just one non-PC opinion. Everybody else's mileage will probably vary.
G.
OK, remove that phrase from what I said. 