I have two darkrooms that I use on a regular basis, one in the central Appalachians, and the other in Manhattan. I have always mixed D76 from tap water at both. Early on, I discovered that I needed to use drastically different development times for each location, for identical film (mostly Tri-x and TMY), exposed with the same meters and cameras.
After eliminating all other variables that I could (e.g., using two Kodak Process thermometers, calibrated together, using fresh d76 from same batch), I was forced to conclude that the pH and mineral content of the two water sources was the culprit.
With adjustment, I can create identical negatives at either location, but the difference is about 2 minutes +/- development time.
I assume that distilled for mixing developer would be standard everywhere and thus eliminate this situation, but I never bothered, just used the time adjustment.
I do, however, use distilled for final rinse in wetting agent, and for the critical parts of alternative processes. I think that if I had free access to unlimited distilled, I would use it for almost everything.
Good Luck and Have Fun!
