In most circumstances, Yes. By buying directly from the airlines you are protected by the Guaranteed Airfare Rule and therefore are eligible for travel credits or vouchers should the price of your ticket drop after it has been purchased.  If you purchase through an online travel agency such as Orbitz, Travelocity, or Expedia you are not guaranteed a travel credit or a voucher directly from the airline.  However, you may contact the agency from whom you purchased the ticket and request that they help you obtain the travel credit.