It's damage limitation guys. I have loads of work that requires internal cleaning, too fekin much in fact. I always use 000 and and by default use a very high flow rate. Now,,,,when I clean the inside I can see spotting almost on every edge of every pane of glass. So.....I recon most of you never really have the opportunity to realistically test your own workmanship. If as I do, you use a high flow rate, and if using 000 tds,as I do, whilst still scrubbing everything in sight so as to avoid spotting, still leaves spotting, all be it lightly, then using purified water to anything less than 000 is a big fat no no in my experienced and well very intelligent great big brain.
In short, if you could see your work when dry and you were utterly and completely honest with even just yourself, well you would be like me I guess....just a wee tad concerned, and to make you feel better, you would use 000 in every case just to make yourself feel better.