I try to avoid working in the rain, because I don't like getting wet; and if I don't need the money; why bother?
However, if I do need the money, I'll work in the rain and have met very little resistance. In fact my customers are more sympathetic and I get more offers of hot drinks.
But all that aside, don't you think that during the Summer when it rains as a 'one off' it's better to give that day a miss, but during the Winter, especially given some of the prolonged spells of wet weather we've had, the customer has no cause to complain?
Okay, maybe some won't like it, but you do remove cobwebs (and they're still about), bird poo and the build up of grime. Not only that, by accepting you cleaning in the rain, they're retaining a good window cleaner.
Other businesses work this way. Have any of you had a childminder and then went on holiday with your kid(s). You still pay for the childminder even though she's not looking after your kids.
It's called a retainer.
I'd say if you want to work in the rain, then work. If a customer moans; drop 'em.