Here is what the Photo-Lab Index from Morgan & Morgan has to say about it:
Th figure of merit for the purity of water is its electrical resistivity; 18 megohms it very pure water. This means the water is not a good conductor of electricity. But the additon of minerals introduces ions that will aid in the flow oelectrons through the soup. The minerals are what gives different bottled waters there taste; Ultra-Pure Water does not have any minerals, and is not safe to drink. The semiconductor and pharmeutical industries are the biggest consumers of this type of water.
How pure does the unversal solvent need to be for holographic purposes? I use the water that the City of Chicago provides me for just about every formula in the photographic/holographic darkroom except for the final wetting agent bath, (Ilford Ilfotol or Kodak Photo-Flo or Photographer's Formulary Forma-Flo,whichever you prefer), as there are some minerals in the water that can leave streaks or dots in the emulsion. Perhaps if I had an in-line filter in the water supply this problem could go away.
But I get by using the Steam Distilled water available at grocery stores, as it is well filtered so that it can be used in electric irons to provide the steam without gumming up the works, or in room humidifiers to prevent chalky build up there. Other bottled waters in the aisle have minerals whihc give them their different tastes.
I use that type of water in compounding the reversal or silver solvent type of bleach used in the Pyrochrome process. Tap water can be used safely in the case of rehalogenating bleaches (Copper Sulfate, Fe EDTA, or PBQ), but the minerals combine with the silver sulfur salts that are a by-product of the silver dissolution, and can precipitate a sludge in the emulsion that just doesn't come out. Sometimes a dunk in a 1% sodium sulfite clearing bath can remove the grunge.
Another case for the purest water possible is in diluting photo-resist developer; or mixing up a dichromated gelatin layer.