i don't know about fan jets but with pencil jets you could rinse aiming just below the seals.
Have you looked at the windows the next day to see if it is really your clean that was responsible?
Mark I am not having a dig at this reply just giving my opiion ok.
if you aim below the sill you will miss the top of the glass leaving problems behind(maybe)
leaching from sills (blacks) is normally due to the rubber disintergrating (spelt wrong I guess)
clean it all, most time if you spend more time at the top a few times it stops, if it does not stop then you have a problem that will not go away, but you can use then what Mark said about avoiding ther top but you will also have to say to the customer that they may find the top of the glass will not look like the rest because of this problem.
That is what I do
Ian