Any time it's been below freezing the night before or the forecast shows it's going that low I ride the studded tires. It's mostly for the black ice in the low areas and places in the shade.

I learned that lesson the hard way on my very first winter-weather test ride before I started commuting by bike (before I had the studded winter tires). The bike hit black ice on a cul-de-sac and I came to in the middle of the street with a concussion.