i clean the top section ,let the drips stop ,then clean the bottom half .
this method does work but is time consuming ,so charge extra.
Not wanting to be contrary but I have a fairly largish job that is nearly all sashes. I've been doing it for years by squeegee then when I switched to WFP, I cleaned them the way that you said. After doing it a few times, I tried an experiment one day. I selected four windows in a row and I cleaned the bottom half as soon as I had finished the top half . At the end of the job I went inside to check and there was no problem with any of them. Now I do them all like that.
It might be troublesome doing them my way on the first clean or two until the frame has been thoroughly cleaned but after that, it was just fine.
Maybe I just got lucky with this particular job.