When I started quilting and building my stash I decided I would always wash my fabric, and so I did it for years, always knew the fabric was pre shrunk and free of sizing. My newer purchases I have not been doing that for pretty much due to all the different thoughts on it, and figured I wanted to save time and money by not doing it. The only way I can tell which is which in my stash, is when I see the diagonal corner clipped which was suppose to prevent fraying whe washing. Wheather it really reduced fraying, who would know. At least it did have the benefit of being an "identifier" as to what's been prewashed or not!