You should always unstring a selfbow, but not laminated bows. Black Widow strongly reccomends leaving their bows strung, as most accidents resulting in damaged and/or twisted limbs is during the string or unstringing process. My personal bows stay strung for months at a time. My main bow, that I've been shooting for 15 years or so, hasn't lost a pound of draw weight--at least not according to the scale.

You can hang the bow by the string, or I let mine rest on two pegs at each end of the fadeouts (basically at each end of the riser, or handle), or you can lay them flat like under a bed. Never leave one standing on end for any length of time, and never leave one where the temps get very high (like in a vehicle in the summer).