If you are cleaning a one-off, or first-time customer, you will have to do it in fair weather. I doubt there's any point in trying otherwise because these customers are expecting a great service and big difference in their window's appearence.
However, all maintenance accounts (monthly, etc...) shouldn't really matter. That's because rain doesn't prevent you from getting the windows clean. Just prevents home owner from seeing the window cleaned (covered with rain droplets).
I always tell my customers that I will make my appointed time whether it's raining or snowing or sunny. It will not effect the results of your service.
Sometimes when raining, they ask: why are you here today!?? I tell them: I really care about my customers...this weather won't keep me from taking care of you. That always works for me.
CAUTION-- For obvious reasons, don't use a pole if there is lightning, eh?