I don't think I've heard of anyone bothering to uncock an RZ shutter for storage. Spring metallurgy is pretty good these days and the tension they're under while cocked isn't very different to that while uncocked.
You don't have to waste a frame of film either. If you really want to uncock it, take the lens off, push the tiny metal button and then twist the cocking levers (one on each side, connected to each other) backwards. It'll take a few goes at figuring what to push/turn at the right time but you'll eventually get it to fire. Don't forget you need to cock the shutter again before reattaching it to the camera, so you'll need to have a body cap to store the body.