Lucky that they plan ahead?

I've got two bikes to commute with now, so I've studs on one and regular tires on the other and ride which ever the conditions call for. When I only had one I would wait for persistent below freezing temps plus precipitation to mount studs; if it was below freezing but dry I wouldn't bother, just be careful around bridges and other known slick spots (it helps to ride the same route everyday, to more easily keep track of changing conditions). My lady puts studs on her bike at the first sign of ice; to each their own.

If I'm in doubt as to whether studs will be necessary the next day, I just give myself an extra 15 minutes in the morning in case I need to switch tires. Gives me something to do while the coffee seeps up into my brain.