I live in an electrically noisy environment. Some of it is my doing - always-on modern electronics with SMPSUs; X10, Insteon, Zigbee, WiFi, and others; and some of it is because I can see big transmission towers on a hill not far away. Plus, my bench gets power through a tangled web of extension cords. I think I share power with a refrigerator; several power bricks; and sundry electrical and electronic devices.
I know that things are noisy because of my limited WiFi range, my garage door opener only works right outside the garage, for example.
In my noisy garage, I have an 8.5 digit Advantest meter hooked to a Fluke 732a reference using coax cable with grounded shields (and separate core conductor for measurement ground and thanks to Bill for showing me these tricks). I am seeing the meter report SD of 300nV across 100 readings. The best one-off I have seen is 130nV. The max-min is 1uV in both cases.
I m not twisting the cables to counter inductive noise, but I do have them tied together to minimize loop area.
It took a few days for the meter to settle down to this level of noise.
Key here is that everything is low noise to start with, with careful grounding and isolated, complete shielding. The shielding envelops almost the entire measurement chain, and has only one ground connection at the 732a. I did not try it with the 732a unplugged from wall power. I am using copper banana jacks, with silver/gold plating.
So I offer this as a demonstration that - with good shielding - environment is not so critical for precision DC measurements.