There are some factors which come into play, ie condition of the seals etc, but this is taken under consideration of how good the window cleaner is and also the technique in which he or she cleans and to that point you should be able to continue cleaning when water is 020 for instance.
The magic 000 is mostly the confidence boosters in salesmen's pitches of old, to prevent any comebacks and money lost to them as they would argue water quality.
Some on here also swear of the magic 000 and never let their water go above, well that is their choice to throw good money into expensive RO's or bags n bags of resin. However a bad technique even using 000 can leave spots.