I am pretty certain they use a bottom sheet, a top sheet and then the blanket/spread. So I think you are ok. They change the sheets every couple of days.
I am not sure if they change out the blanket/spread after each cruise or not. I think they do, but I have never hung around long enough on the return to see for sure.
I'm sure someone knows....
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007