In general, it is not a safe practice to dry-fire your crossbow.  Your crossbow is equipped with a DFI (Dry-Fire-Inhibitor) mechanism.  In the event you forget to load an arrow into the crossbow but attempt to take a shot, the string will be caught by the DFI, which helps to prevent damage to the bow assembly.  However, the force of the bowstring hitting the DFI lever can sometimes be enough to cause the center serving to fray or break.  After re-cocking your crossbow and taking a shot, inspect the condition of the center serving on the bowstring.  If you see any strands broken in the bowstring or in the center serving, have the string and cables replaced on the crossbow prior to shooting it again.