As usual I buck the trend and use the defrosters all the time. If the car was used recently the glass has already been warmed by the defrosters so if it was going to break it probably already would have, and if the car has not been used recently the defrosters are going to blow cool air at first and gradually get warmer. That's about as safe as you can get in my opinion and I have yet to break a windshield using the defrosters. I also warm from the outside in most cases with a hair dryer, and under a bubble dome when possible, but using the defrosters helps maintain the temperature and controls condensation.
Like GlassStarz I've done plenty of repairs in below zero weather, but since moving to Oregon have rarely had to work in anything colder than 30F or so. Hopefully my next move will up the minimum temperature to about 50F or so. We are really spoiled any more as we do most of our repairs indoors. We have customers in Siberia and Alaska who do repairs all year round, but I have to assume in the really cold weather they figure out a way to get the vehicles inside. I've worked in the oil fields at -50F and it ain't no fun!
I keep my resin warm by keeping it in the vehicle and use the hair dryer if additional warming is necessary.
How cold is too cold? That depends on how hungry you are.