Sfvoyage, Premier & other room-night cert.s are totally ineligible for anything--stay credit, night credit, points, & FFN--earning. But every so often someone gets lucky.

Dollar-amount gift cert.s & cards are eligible for everything (except FFN-earning??)
If U wanna try, I would recommend using them at the beginning. From another thread, I believe that we will earn FFNs if the MasterCard charge is the last item on the acct., which brings the balance to zero.