For the past five years, I've just waited until snow flies. As I recall this has ranged from around Thanksgiving to mid-December. Changing tires doesn't take any more time than fixing a flat, so I don't worry about it until the weather dictates.

I was out this morning too, (28°F here) and saw only a patch or two of ice, always in places where there are typically puddles. I know the roads on my commute well, so until there's snow and ice everywhere, I can usually anticipate and avoid little patches like we had this morning.