I've personally never had it fail on me. For a while I thought maybe there was something not quite right about how I was mixing it (using either deionized or distilled water) because my development times with TMax films were always longer than Kodak's indications. But no matter what I changed I always got the same results so I stopped worrying about it.
I've never had an XTOL failure either. Steam distilled water is sold here in grocery stores at $3 for a 2-1/2 gallon container. Compared to the cost of everything else in photography, that's noise level. I see no reason not to keep using it for mixing/diluting XTOL and other powdered developers, diluting liquid developer/fixer concentrates and mixing Photo-Flo rinse solutions. Ultra-cheap insurance, especially where film is concerned.
I do the same. I don't know if it is truly required, but whatever.
Most "deionized" water sold in this area is actually produced reverse osmosis. Try putting it in your car battery and see what happens. I'll stick
with distilled, thank you.
We use in-house deionized water in the laboratory I work at. One thing to keep in mind with DI-water is that it will rust and pit stainless steel over time. Because the mineral ions are removed from the water it tends to pull ions off the metal, resulting in the pitting.
I've only processed a couple of rolls of film so far since I'm just restarting the whole film process, but I mixed up my ID-11, stop, and fixer as well as washed with filtered water from the tap with no issues that I could see. We just have one of the under-the-sink RO systems that removes most of the minerals. I figured I'd try that first before having to buy and store bottles of distilled.
To clarify my post on distilled water. If the condenser portion of the distilling apparatus is made of metal such as copper or iron this will introduce these ions into the water. Ascorbate is very sensitive to these ions due to the Fenton reaction. This is why Kodak uses a chelating agent DTPA in Xtol
It's not smart to use lead solder in your moonshine still either. But correct distilling is still going to give the cleanest water. Reverse osmosis
is for removing organic contaminants to create drinking water from whatever; some chemicals might get thru anyway. It's a semi-permeable
membrane system, to phrase it generically. Or you could passive filter things with a portable water purifier system - still wouldn't remove
everything. A lot depends on where you live and what the tap water is like. Here we get good snowmelt from clear across the state, so the
water quality is excellent. A few miles inland they get treated river water that is essentially over-chlorinated swimming pool water with a cocktail of agricultural runoff, incl trace amts of all kinds of pesticides. Dry climates can have very hard water with quite a bit of alkali. I hesitate to say where some of our highly advertised bottled water comes from. A least one brand is bottled at a superfund site, because nobody is actually allowed to live there anymore! The spring is some distance away ... but it still makes you want to think twice!