I am trad, and live in the south west of Ireland, where it also rains a lot. I usually stop when it rains, as most customers don't feel it's worth it. Some appreciate that I need to make a living and let me continue.
My thoughts are if the rain is light and not hitting the windows, there's no problem. If it is heavy and/or it's hitting the glass, I stop. In some instances, the job has to be done regardless. In those cases, this is what I do:
1. Wipe frames with microfibre cloth/scrim.
2. Mop and squeegee glass.
3. Carefully wipe edges and corner of glass, taking great care not to allow any part of the cloth/scrim to touch the glass anywhere else.
3a. If the frames are flush with the glass or are the old wooden type, ignore 3.
4. Wipe sill.
I use a damp cloth/scrim for the frame/sill wipe, and a clean dry one for the detailing.
hth,
John