If it's a drizzle to light shower all day, we work. If it's a constant downpour we'll likely reschedule.
I tell my customers, if we rescheduled every time there was a chance of rain we would never work. I also let them know if they think any windows got spotted or streaked once the rain stops, just call us up and we will touch up any of those windows.
Maybe one or two times a year I'll get a callback to touch up, and usually it's a blown seal not even from the rain.