Most coops need ventilation more than being "draft free". I wouldn't think much of a draft will happen there, but I can't say for sure. If there are large gaps on the side of the prevailing winds, and they are up where the chickens will be roosting, I'd close them off, otherwise leave them be. You can always lean something against the outside during the worst part of winter if you think it's a problem.
just too add on, you can always use weather stripping if you are just patching a few big gaps......