I prewash all of my fabrics before it goes into stash storage. Sometimes if I purchase material just for stash, don't have enough for a load and don;t need to use right away, I'll put it aside in my "need-to-wash" fabric pile, and once I buy more I wash them together (like-colors). This way, all of my fabrics are ready to be used.
I know many people don't pre-wash their fabrics, it is more of a personal preference.