As far as I know, the only time you need to unstring your compound bow is when you're replacing the string, or doing some other repair. If you have an older compound bow that's made from laminated wood, then you'll want to back off the tension on your string when you store it, but modern compound bows have limbs that are rated up to 200,000psi, and can be stored for years while strung without any problems.
It's your strings you need to worry about more than your limbs. Make sure your strings are properly waxed and store your bow in a cool, dark, dry place, preferably in it's own case. One tip I heard once was to tighten your strings all the way to the max, then back them off about 10 lbs (~2 1/2 turns) before putting your bow away into storage.
Not all bows are the same, so it may be wise to consult the user manual for your bow.