Not all torque wrenches require it, but chances are if you own a value-priced click-type torque wrench like this $30 beauty from the local discount auto parts store, then you should get in the habit of de-tensioning it. Once you've cranked it up to whatever torque setting you need and tightened the targeted nuts and bolts, simply de-tension it by lowering the torque setting to near zero so that the spring mechanism inside the torque wrench is not left in a compressed/loaded state. Failing to do so can result in a torque wrench that loses calibration and no longer provides predictable and consistent torque measurement.
Also keep in mind that your torque wrench is a precision tool that should only be used for tightening fasteners. Don't use it as a breaker bar, since putting this kind of excessive load on it can also upset its calibration and break its ratcheting mechanism. Just like grandma told you, don't abuse your torque wrench!