I have to say I agree with Vince. I've dumped plenty over the last 3 years as he has said he has, but I sit at home and wait it out if its heavy rain as it is here today - lashing down. I reckon i'm in the customer satisfaction business, rather than the window cleaning business, and Vince has hit it squarely on the head - they might let you clean 'em but are they happy that you are doing it. Probably not. You could argue that if they don't say anything then it's fine to go ahead, but to be truthful I just don't like it.
If it's light rain, on and off i'm out there, but not moderate or heavy rain. To an extent it also depends where they are. If on a main road where traffic spray is gonna be on them within 10 mins of me finishing then it's fair enough that they don't want me when it's rainy - if it rains the next day, then fair enough. If they are on a quiet estate then even heavy rain shouldn't make a difference.
It seems to work for me, plus I stay dry(er) which is more pleasant. I seem to catch up as I leave a few spare days in the month anyway for bad weather and still make a great profit.