Resting a turkey (or a steak or a leg of lamb or even a large fish) is absolutely necessary -- not optional -- for maximum flavor and juiciness. Keep in mind that during the resting period, the turkey will keep cooking, and the internal temperature will rise by 5 to 10 degrees. Pull it out as soon as the temperature hits 165, or even a little lower.
The amount of resting time depends on the size of the bird, but at least 20 minutes is needed. A large bird can wait up to 40 minutes or longer, depending on the temperature of the room. Cover it with foil, but loosely; you don't want to seal in the moisture, which will cause the skin to steam and lose whatever level of crispness you've achieved.
