You don' t want to twist the string just for the sake of having twists in it. It should be twisted to the proper length (under tension). Adding more twists then needed to get to proper length, will change the tension the limbs are under and your draw length. Most custom string makers take this into account and allow for a reasonable number of twists. Most machine made strings, creep to the point where you continually have to add twists to keep things the same.