Yes, WiFI disconnect should be ok. After boot or after deep-sleep the wifi is disconnected and disabled until you enable it (WIFI begin).
In modem sleep I think that WIFI is also disabled.
So, what I do usually is first take the measurements and whatever business it has to do, and at the end connect to the wifi.
I suppose that the wifi signal is a relative strong electromagnetic wave that would add noise to the voltage measurement. And it can be emitting not only when you send something, but at any moment if it's enabled.