I've never offered one in writing but I have verbally.
I just explain that regular rainfall prevents a build-up of dust in the atmosphere so generally speaking most rain isn't that dirty. However, if they are dirty tomorrow, give me a call and I'll re-do them for free.
This wouldn't present too much of a prob for me as most of my work is close together.
It does seem to put the customers mind at rest if they inquire as to the point of cleaning windows in the rain or if rain is forecast.