i do the all the frames first, including ones with the vents, but give them a really good rinse, then come back and do the glass, by the time you have done the frames of the whole house then gone back to the first window it should have stopped dripping from the vents.
On first cleans, if the customer is in i just warn them that there could be a problem with spotting but there shouldnt be, but if the is and they are not happy then to call me and i will redo them, but dont make a too big a deal of it with the customer or it may worry them and put the off wfp.