most of the time they will be fine, we clean in all weathers and have not had a complaint, make sure you do a good job, most of the time the rain never hits the windows full on anyway. Some people will say no as they dont like getting wet, and if we had a Summer like we did last year I can see these people going out of business or changing there thoughts on cleaning in the rain.
A good tip is dont listen to the met office most of the time they are wrong, I must have lost a shed load of money when I was trad watching the forcast from them, take each day as it comes get out there and clean would be my honest opinion.