Distilled water is cheap enough in most of the US (I understand this is very different in the UK and elsewhere) to just not be an issue. It's $0.79 a gallon at the local Kroger. It's a bit more at some other local grocery stores but still less than $1.20 per gallon at worst case. I routinely use it for all my developers even though I've seen no indication that I need to, just because it's cheap insurance. I haven't used it for stop, fix etc. because my belief has always been that those are less critical. I do mix Photo-Flo with it for my final rinse.