I talked with a paper conservator from the Baltimore Museum of Art over the weekend about using buffered tissue/paper in the storage of pigment-dye, cotton-rag-paper prints. He said that the safest thing to do is to use unbuffered paper, but to not worry if you're using buffered paper. Apparently some relatively recent research has shown that buffered paper was not harmful to C-prints after all (in past years some had claimed that it was). Also, he can't think of any reason why buffered paper would be harmful to pigment-dye, cotton-rag-paper prints.
-Scott