However, is working in-between the rain storms any different?
no difference at all, when i finish and it hammers down, i allways think, well that was a waste of time
But it's not really is it?
You've removed bird poo, spiders webs and stuff. My windows haven't been done since the last time Wor Lass forced me to do them and they still look pretty good and they've been rained on countless times since then.
Rain usually doesn't affect the windows; though I admit there's factors affecting, for example I do a few houses that have been painted with cheap paint, and when it rains the water runs down the house and some of the paint residue ends up on the windows.
You may also have patio doors (French windows) where the rain splashes dirt from the ground onto the bottom area of the windows; causing them to get dirty.
But in general, rain does not make windows dirty!