The answer varies. Firmware version 114R (and unpublished version 115R) were the last versions where the "flat" remote buttons (particularly the CC button) functioned as designed. The next version, 124R, had some other "surprises" too, such as removing the ability to delete unwanted channels; so some folks stayed on 114R because of that.
124R was such a big change to the way the DVR+ worked, many folks changed their attitudes toward CM firmware updates afterwards, and began upgrading only if they wanted a feature added in the new release. Some even went so far as to stay on 132R when 134R came out, even though 134R was merely a bug-fix release, with no "surprises" I'm aware of.
Also, version 124R was the only version that could format (some) HDDs over 2 TiB to use the full capacity of the drive. All other versions have a 2 TiB limit. (Of course, you could format your HDD with version 124R, then upgrade; the newer firmware versions will still use the full-capacity format created by 124R. They just won't format it to full capacity in the first place.)
Version 132R added Sling TV, which no longer works on any DVR+ firmware version. However CMTV no longer works with 124R. Also, 132R fixed some of the mess 124R made with the colored buttons on the remote. So unless you frequently format large HDDs, I saw no real compelling reason to remain on 124R. (In a pinch, you could always downgrade to it with a mantenance restart.) But everyone has their own reasons. Heck, a lot of folks still use Windows XP, and I see nothing wrong with that.
I stayed on version 134R mostly to see how long the guide would keep working! The answer turned out to be 9/5/2018 (two weeks before the 9/19 end of the last Rovi guide). So surprisingly, earlier versions worked for about 9 months after 135R was released.