Just a word of caution. I used Gift Cards on a recent cruise and our small surplus was NOT refunded to the Gift Card. On the account we had: OBC from Disney resulting from an OBB; OBC from our
travel agent; we used some Disney Rewards dollars and lastly placed several Disney Gift Cards on the account. When I checked the gift cards to see which one had the surplus, all of the gift cards had a zero balance. I inquired with DCL to see what happened. DCL informed me that the surplus was refunded to my travel agent (who had placed a "refundable" OBC on my account). I inquired with the travel agent who advised me that DCL policy is to refund any surplus to the first form of payment, not the last, and that the policy is that OBC is not refundable. That seems odd to me, but that's what I was told.
This has since been resolved to my satisfaction, but I have also been advised to use any Gift Cards late in the cruise and for the exact amount currently on the account (i.e., do not place large sums on in advance unless you are confident that you will use it ALL).
My experience may be an isolated one, but I wanted to post it since I frequently see others post that remaining balances are put back on the last form of payment used. YMMV.