Basically this is what I tell my customers.
I clean in light & medium rain. I don't clean in heavy rain as I'm risking the flu and a week off work. If it rains whilst I'm cleaning your property or on the day of your clean and you are not happy with the quality, give me a call / txt and I will come back and re-clean the windows free of charge.
I would rather come back and clean them as your windows are my advertising boards. If you should ring, I don't bite, not even if you ask me to.
That's what I tell my customers and not one has called me back after cleaning in the rain