I would add not to leave them with the shutters cocked, especially if they have cloth shutters. Over time this will stretch the springs and throw the shutter speeds out of adjustment.
I think this has been shown to be mythology as the difference in tension in an uncocked spring compared with a cocked spring is very small. i.e. in an uncocked state the spring is still in tension, probably around 80% of the cocked tension, in order to get a more linear response. If the spring went slack at the end of the movement there would be a huge difference in tension across the travel of the shutter resulting in it slowing down towards the end of its travel.
"People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.