[Londoner]

In actual fact water from above drips down for a very long time after the window has been cleaned, whether you brush the cills of not.
Thats why I have tried to develop a technique which is not the most time efficient but leaves the max time before you do the windows below.
Its no good going round the house doing top - bottom - top - bottom
Potentially, depending on the house it can be a significant problem. If the water drips down and splashes on a cill it does cause spots. Wind blown drips cause runs