I now have the luxury of two wheel sets for my winter bike, one with an aggressive tread and the other with studs. That makes swapping them quick.

Before I got the second set of wheels it was a different matter. I would change tires if I had to put the studs on in the morning before leaving for work. Otherwise, if I had the studs one I'd usually just wait until the weekend to see Sunday night what the weather looked like. During the week, unless it looked pretty certain that the temperature was going to be high enough I wouldn't need them, in that case I'd change them at night when I had more time.

I'd rather ride with studs when it wasn't necessary than wish I had them on when it was. Remember it can be hard to see ice in the darkness or if it's waiting for you around a corner. It only takes a fairly small patch of ice in the wrong place to cause you problems if you don't have the studs on.