Hello... We just got back from a week at the World and prior to going we bought $700 of Disney gift cards Sam's (they sell $150 worth of gift cards for $142... you get 3 $50 cards in the pack) and they were accepted at all the locations we used them at, including pick a pearl at Epcot in Japan...
If you are staying on property and for whatever reason a store does not take your gift cards you could always just charge your purchase back to your room and then go down to the front desk and pay your charge amount off using your gift cards...