Some of the bowyers, such as Black Widow and Bob Lee, recommend leaving their recurves strung when you're using them regularly. This would be for modern fiberglass backed recurves. Apparently the risk of twisting the limbs through repeated stringing and unstringing is greater than the risk of losing poundage through leaving the bow strung.
Black Widow recommends unstringing their longbows when you're not using them, which I would assume is because the risk of limb twist is much less in a longbow than a recurve. However, if I was going to use it later on that day, I would leave it strung, and unstring it when I put it away at night.
A self bow or any bow not backed by fiberglass should be unstrung whenever you're not actually shooting it.