Yes, the U.S. should take North Korea's Saber-Rattling Seriously, because the leaders of North Korea think very highly of themselves. If North Korea's leaders are capable of damaging their own people and causing large amounts of suffering, they are capable of trying to inflict damage on the rest of the world as well.
Although North Korea threatens the West frequently in the media, they are not capable of doing any real damage. North Korea is highly impoverished and most of their citizens are starving. The country would not be capable of a major offensive against a major industrialized country like South Korea or the United States.