Winter tires on all four wheels in regions where the temperature falls consistently below zero.  I would not take them off until the temperature is consistently above zero.  In my region, that would be from mid November to mid April. Drive according to the weather conditions at all times.  Be aware of driving conditions early in the morning heading to work.  The worst conditions of all are black ice, slushy road conditions, the transition from rain to snow.  Four winter tires will make your vehicle a tank.  All season tires are, in fact, three season tires.  

I write from experience having driven on some of the most treacherous roads in Canada-  Highway 16, the Alaskan Highway, Highway One, the 401, in compact cars.  I have had two near death experiences driving all seasons in winter conditions.  Never again.