If you look at this frost (plant hardiness) map it generally tells where you need winter tires and where you probably don't....
In the blue zones you should really consider winter tires. In the green zones you should probably get winter tires. In the yellow zone you probably don't need snows, but also good all seasons (like the Conti DWS06) would be preferred as temps get really cold in the winter and summer tires may be an issue, but it wouldn't hurt if you are really safety conscience. In the orange and red zones you don't need winter tires for those few times is gets below 40*, but all seasons should also get you by.
Any time that it gets below 40* at the same time it's wet, summer tires aren't the best choice.
--kC