Dunno, but I wouldn't like to leave my guitar out in the shed at the minute. As far as I'm aware, it's extremes of temperature or sudden changes that are the main issues.
I would think that if you insulated it fairly heavily (which would be good for sound too) you may be OK, but I would wait for someone else to confirm it. I'd still bring them in over winter though.
Also, concrete slabs for garages are often right buggers for conducting heat straight out a room, you'll definetly want to insulate that as well as the walls.