My old, and new, flares live on the boat. I don't think it hurts them to be exposed to the cold as I've fired some long-expired flares and they seem to work as well (or as poorly) as newer unexpired ones. Having said that, I think it is important for all flare owners to (1) find a legal way to practice with them ashore (i.e., 4th of July or a fire fighter training program, etc.), and (2) keep something like an oven mitt with your flares in the event you need to use them afloat. A mitt will help protect your hand from hot drippings or overflow from a burning flare, which I think is somewhat common.
Also, remember that if you have an orange distress flag aboard, and do not sail at night, you may not need flares at all. It all sort of depend on where you sail and the other forms of emergency communications you have on board (cell phone, VHF, horn, whistle, etc.).