For general all purpose chemical dilution and mixing, I use filtered tap water, except for doing colour. I have always used Demin water for the final rinse when developing film. I have found that this is the best way to ensure clean streak free negs. I had thought of using demin for the whole process, but the price of buying 4 litres at a time has put me off.
I believe Polyglot uses bottled spring water (for the chems at least), but I believe his tap water is really quite bad.