Hi mate,
Do you KNOW you can clean window to the same standard as it the dry?
If the answer to that question is YES then carry on.
Explain to your customers that you could clean thier windows in the mornnig then it pours down in the afternoon, it's the same effect! at least you are cleaning a months muck of and keeping thier windows clean regulary. If they can't accept that then don't do them.
In 16 years in this game I've learnt you have to set YOUR stall out, do what YOU believe is right and fair.
The only thing with the rain that stops me is when I am getting soaked. My mortgage payment don't stop if it's been raining!!
David Salkeld