Rick, I've personally attended sessions with six different MEs doing mixes of mine. At no time was widening done, or even brought up.
The only problem I've personally encountered with continuity had to do with a tape machine misalignment, and one track was significantly brighter than the others. We fixed with with the Tube Tech multiband comp.
If I mix one song "narrower" than the other songs, it's because that was my intention. There are plenty of classic records that have examples of that, and nobody has ever complained.
Personally, I've never been a fan of M/S widening, in any degree. Sometimes there are problems of mono cancellation. But most importantly, when it comes to mixing, sometimes I'll do a trim of an instrument of just .5 dB or .25 dB to get something to sit exactly where I want. How much does your 1/4 of what everybody else does change that level? My decision to place that instrument precisely at the level that the artist and I agreed on has now been usurped. The artist and I may have spent months making this record, and getting to that final result. Now somebody who is spending a total of 4 or 5 hours on the record is deciding things for us after the fact. I'm sorry, but in principle, I am against that.
Also, I encourage all MEs to go into mix forums and school mixers, particularly on the use of limiting, with the threat of violence. You must feel like that when you get a track for mastering, and it looks like a stream roller has visited it. I had a famous ME add distortion to a record I did (unintentionally, of course). Violence certainly crossed my mind!