Topic on here some time ago, I mentioned I've cleaned with just RO water, and it was fine for the majority of houses.
I was using around 9-10ppm. Was doing this for about half a year when using cold. What I have found is it leaves spots behind when using hot. Also, di-ionised water at 000 cleans much faster I found. Quicker with rinsing. Don't ask me the science, just seems that way. Perhaps absorbs dirt more readily. Have cleaned water till about 27ppm, but strangely enough needs more time rinsing.
Not saying you can't clean with just RO water, but not as quick.