Lots of rollyeyes........
I've worked with tds between 10-20 in direct sunlight and it has indeed left spots on the window.
17 would be fine on an overcast day, not near the sea on a maintenance clean.
Wouldn't risk it on a first clean though.
And yes - the cutting ability of DI water reduces exponentially as the tds rises.
Do a bit of research on how DI water cleans - it really is very interesting 

Oh right, so direct sunlight causes the spots now then? So, if it clouds over my water wont leave any spots?
Try cleaning a window with tap water- it cleans exactly the same as with pure! apart from what will be left behind of-course. Cutting my ar*e!

I've heard some tripe in my time on here!

Do you actually have any idea how WFP Cleaning works?
0 TDS water doesn't exist in nature. By modifying water using a mixed bed resin you are altering it's natural state.
If you look at the Planet we live on, that's Earth in case you missed it, anything modified or altered beyond it's natural state will always return to it's natural state.
0 TDS has 'cut' because it will consume anything it can in order to return to it's natural tds which is obviously considerably higher than 0.
As the tds goes up and the water is no longer pure contaminates are left behind in the droplets.
You may not have noticed this but if the Sun is shining on a window the window gets warm - sometimes very warm.
If the window is very warm the water will evaporate very quickly and the contaminates will then dry on the window leaving spots.
If you're in direct sunlight this makes the problem much worse.
If you're on the opposite side away from the Sun, that's the shiny yellow thing we see sometimes, the window is colder and evaporation takes place much more slowly
meaning you can probably get away with a higher tds.
DI water that runs off a sill and over algae will actually consume it. Tap water just turns it slightly green and may remove some of the loose stuff but that's about it.
Di water has a ph of around 6-6.8, sometimes in the 5's, which makes it mildly acidic and this is probably where the cutting element comes in as it will balance out to 7 once it's fed. Have a read of these, they explain it well:
http://answers.yahoo.com/question/index?qid=20081030065718AAlWBsKhttp://www.lenntech.com/applications/process/demineralised/deionised-demineralised-water.htm