The technique I use on a first clean is to give the frames all a good run round with the brush (on a section of the house, say do the whole front) and then by the time I'm back at the first window the vents should have stopped dripping. pay good attention to dividers as dirt will collect on the top side of them. then move onto the glass giving each pane a really good scrub making sure to get along the tops of the frames where they meet the glass, go right round the section of the house and then back again to the first window and do the normal for my maintenance cleans which is two agitations and a good rinse. Then move onto the next section of the house and do the same.
It's worth putting in the extra effort to keep the customer happy and ultimately keep the business. After a first clean I do explain about the build up of years of dirt/detergent which can cause the odd run but tell the customer I've cleaned them 3 times and they should be perfect but if anything they aren't happy with, just text or ring and il happily put it right. Never had a call yet.