First, metal springs do not fail if left under tension unless the ambient temperature is about half the metal's melting temperature. (If metal springs could fail in this way, then the beams in steel buildings would fail from decades of load. And this also explains why modern steel buildings coat the beams in fire resistant foam -- if a fire heats the steel to half it melting temperature, it begins to slowly sag.)
Second, however I can imagine that some designs of older cloth shutter cameras might leave the cloth under tension and that the cloth might stretch. This effect would only happen to some designs.
EDIT: Not a Number beat me to it!