In my opinion, you can end up on a slippery slope with stuff like this because it's easy to get caught up in all the specs and numbers. Before you know it you will be sweating over which reference works best in terms of ppm in various scenarios of time and temperature etc...
In my case, I've not adjusted any of my DMMs in 15 years or more. I do have some cheapo voltage references here and whenever I can be bothered to check my DMMS they all seem to be boringly consistent. The 3.5 digit ones obviously have limited resolution but they seem fine year after year.
I do have a 6.5 digit DMM here (Keithley 2015 THD) but I bought this for the onboard AF DSP to measure SINAD and also for the onboard AF generator. I really don't care how accurate the last couple of digits are.
However it does seem consistent with my old Fluke 45 in that it agrees within about 1mV at 5V and beyond that I just lose interest.
You asked for opinions (
) and I just don't see the thrill or relevance of this stuff. DMM technology was boringly brilliant over 30 years ago and few people need this level of performance even today (myself included).