wally the pics on your site are great. do you use wfp on polycarb roofs?
I try to be as economical with my pure water as possible. If there's a tap available and I can use the customer's garden hose, I'll use tap water for the entire job - top and sides. Then, I'll finish it off any glass with pure.
If there's no tap or it's otherwise too much messing about, I'll use pure for the entire job.
And I agree with mlscontractcleaner, wfp does make the job a whole lot easier. It's just that I'm not too worried about it taking a little longer with tap water and then pure. Others would be.
You could quite easily start off your business just using tap water and finishing off the side windows with a mop and blade. As you get spare cash, invest some of it in an RO setup and go the whole hog.