I've cleaned with a high TDS of 35 before and it looked good in normal light, but when the Sun was low in the sky and the light polarised the marks on the window that were invisible earlier; it didn't look so good.
I'm sure I've seen Stu post similar; he lives in a soft water area which has a naturally low TDS (like 25 or something), yet he still puts it through a DI, and he's a tight jock, so if it wasn't required, he wouldn't do it.