You can leave tension on springs and nothing will happen to them.
Perhaps some creep. But before you notice any effect of that, you'll children's children will be telling stories about the good old days when they were young to their children's children.
So whatever you decide, before or after, don't decide to go with 'before' because of fears about what keeping a spring tensioned might do. For it does nothing.
Spring tension does nothing to the metal they are made of, unless you put so much tension on them that they break. So it doesn't matter either at what setting you keep them.