After reading some of mawyatt's reference articles and re-reading this thread a few times, I conducted my own tests on every DMM I could lay my hands on over the past week. In every case, mawyatt's results were the type of results that I found independently.
However, I used a CD4047B as my square wave generator, set it at 100Hz to mitigate 60Hz EMI and environmental influences, powered by an MC78L15 A-grade voltage regulator, which also powers my LT1236A-10 10V Reference IC, and added a couple of precision resistors to the PCB and came up with a "poor man's" version of a DMM checker.
The damned thing worked so well and so consistently that I built another one and listed it on eBay to see if there is sufficient interest in this type of device to refine it and offer it at a more affordable price along with my sine wave references and precision 10V reference modules.
Here's an eBay link for this all-in-one 10V reference, square wave calibrator and Ohms checker for your comments or suggestions:
https://www.ebay.com/itm/285825801852Remember, this is only a prototype so you'll probably have some criticism, etc. which I appreciate, as always.
No spamming intended here. Just sharing my experience showing that the square wave DC functionality test seems to be pretty bulletproof with popular DMM's in all price ranges now.