First time I see frost on a window anywhere or piece of bare metal. Means we're probably going to have some nasty weather day very soon like freezing rain, a hard freeze the night after rain, or snow which melts just in time for a freeze overnight.
I use studs front and back. I'd rather not go down in front of a truck because I was being a cheapskate. I leave them on all winter; if there's no ice in sight, i'll just ride on the studs. I'm slightly slowed down, big deal! I'll be a stronger rider when the leaves come back for it anyways, and I already planned my trip times around the studs.