I don't like to work when it is any colder then -15 F. I'll work in the rain as long as it is not coming down too bad. Obviously no thunder and lightning and wind play a factor if I'm doing a difficult removal.
If I even see green on the radar map, I knock off. Don't work with snow on the ground, not worth it, labor increase eats me alive and I don't like to work just to work.
I think it really depends on the job and what equipment you have. Otherwise the question is to vague. Deadwooding a large pin oak in the rain? No way if its a climb. Doing some take downs next to the driveway or street with a bucket? Or with a crane or hopefully both I would def work in a light rain. I also pay close attention to not chewing up peoples grass so it just depends.