TDS well over 400 here, often use customers tap to clean filthy windows and conny roofs etc, then use the pure to go round them after for rinse and minor scrub.
Never had a problem, just don’t let them dry after using the hard water for too long.
I also use customers taps for gutter and facias cleaning.
Makes sense to me, I don’t like to be using pure when I don’t need to, obviously if it’s a first clean that’s not too bad, then pure water from the start, but if it’s absolutely filthy, woth filthy sills and frames as well, then the customers tap is my go to