0ppm or 10ppm it doesn't matter, as long as the window is cleaned and rinsed properly the results should be good.
Around 25 ppm is the point you start getting spotting, I know that as a matter of fact.
A company I worked for many moons ago used resin only to filter water with a tap TDs of 349ppm. On average they would get 3 days out of an 11 litre vessel, changing at around 25ppm. We never had any issues, apart from the odd occasion the water crept above this point to 30 odd ppm, then the complaints came.
I personally prefer to keep my water under 3 ppm for peace of mind more then anything.
If anyone gets bad results with anything under 20ppm then 90% of the time it is through poor cleaning technique, i.e,. Not scrubbing properly, not rinsing properly, poor flow, not enough care taken around vents and/or badly eroded rubber seals.
The other 10% of the time is because of environmental issues ( wind, pollen, sap, pollution, dust from nearby building work) or issues with the glass itself ( limescale, blown glass, poor quality glazing unit etc).
Pilkinton glass is the worst, it looks awful in sunlight no matter how well it is cleaned. A local glazing company has stopped selling pilkinton due to the high number of complaints from these units. In direct sunlight they look really spotty, but it is usually particles between the glazing that show up more so then any other glazing.