It is a mystery to me, how some windows that are really dirty will sheet, only to start beading after a few cleans.
When I first went WFP, and was able to reach windows over cons that had not been done fore years, some of them sheeted perfectly for a couple of cleans, then started beading. WHY?
If windows bead, and the beads stay put until they dry, there is more chance that airborne dust can stick to them and leave spots. I believe the real source of most spotting is the frames, most spots are right next to them. The more attention you give to the frames the less spotting you get, it's a question of how much time you can devote to the frames, and some frames are a lot easier to clean properly than others.
If you want perfect results on all windows, the frames may have to be considered when pricing the job. Dai