I unstring my bows when not in use (though I might leave one strung if I know I will be using it the next day).
That said, I once heard that Martin took one of their traditional bows, strung it, checked it on a bow scale, and then hung it on a wall in the shop for a number of years. As the story goes, when they finally took it down they put it back on a bow scale and again checked the draw weight. Supposedly there was little if any loss.
It would be really interesting to find out if this story is actually true.