With your weather it would be smart to have a heat source *available*, in case of need. Whether you will actually have to USE it often or at all depends quite a lot on your particular coop situation -- how large, how constructed, how ventilated, whether you can keep it real dry in there, etc.
When it gets real cold, put plenty of depth of shavings (or whatever you use for bedding) on the floor -- even a foot is by no means excessive -- make sure they have a WIDE roost as mentioned in an earlier post, and never ever let them run out of food. Do not shut the ventilation off in hopes of keeping things warm, it just causes humidity which causes frostbite. For large single-combed breeds like some of yours, it can't hurt and may help to put vaseline on their combs a couple times a week during the coldest weather.
And keep an eye on them. If you see signs that they are stressed, or the first earliest touches of frostbite, consider running a lamp -- not intended to heat the coop and it need not *necessarily* even be high-wattage or a 'heat' lamp, a regular 100w bulb can often be quite useful, you just want something they can hunker down under if they're feeling cold. Obviously it needs to be safely rigged so it cannot fall and start a fire and so they cannot bonk into it and burn their combs or break the bulb.
But since the point at which chickens start to experience problems from cold depends on so many factors, really YOU (being there) are the only one who can tell when and if it makes sense to actually USE that supplemental heat.
Good luck, have fun,
Pat