Last year, I rode through the winter, a few times each month. Generally, though, I do my first real "ride" of the year between March 5 and March 15. Sometimes, there's still some melting snow on the side of the road. I am very leery of this, since I no longer have a hybrid or mountain bike to deal with water, sand, etc. It really does depend on the conditions of the roads. I also am lazy about cleaning my bike, so I don't want to put myself in a situation that will need a major cleaning.
Since my good friend broke his hip on black ice on a morning in March when the temperature was going up to 60 for the commute home, I tend to pay more attention to these things.