Alan, welcome to the party. I live in Calif but I would say from others stories, that it is not necessary to use the jacket at any temp. However logic states that you will maintain your smokers internal temp better and use less coal if you use the blanket. Any other device that you create to minimize temp fluctuation and loss will benefit. There have been those who use tents, wooden shacks, and windbreaks etc.
Happy Smokin' all winter long.
Mark