TDS is the impurities in the water. I have heard it said that different water contains different impurities with the same TDS.
From personal experience, we can not clean windows with anything above a TDS of 10 as the windows spot and are noticeable on close inspection. As has also been said, sunny days will show up spots that you may not even see on a close inspection from outside.
When cleaning a hot window, you have to cool it down first so the water doesn't dry before its flushed the dirt off the window. This was first stated by some WFP cleaners working in Cyprus many years ago. With double glasing, this also means reducing the temperature between the windows.
I've also experimented with a drop of Ecover in a backpack and got spots remaining - left by all those lovely bubbles.
So for us, its a TDS of 0 and never above 2.