A direct quote from someone who knows, Bill owner/maker of Excalibur Crossbow.
You don't have to unstring your crossbow every time you use it, but it will appreciate having its limbs relaxed at least between seasons when you aren't using it. The #1 concern is that you NEVER leave the string on if your bow is exposed to high heat, such as in the trunk of a car in the summer sun!
I unstring mine unless I'm shooting almost every day, and unstring if temperatures are very high (like over 75-80F) where I store it. In the summer I just target shoot without the DB pads, so I don't have to take them off each time I unstring. When Deer season gets here I will leave it strung with the DB pads on & ready to go.
I unstring when I am finished deer hunting, usually sometime in Nov. It stays that way until late March or early April when I start shooting on a regular basis again.