Washing fabric prior to using it for a sewing project is a requirement if you ever plan to wash the finished product. You run the risk of your finished project shrinking substantially and unevenly after the first wash if you opt out of prewashing. Never plan to wash your finished project? Don’t bother with prewash.
I enjoy the variety of fabrics that come in fat quarter bundles. As I’ve worked through beginner projects, fat quarters have often been the perfect size for small projects. I’ve made the mistake of washing 10 fat quarters together without pretreating the edges. I pulled a knotted mess of fabric out of the drier.
I recently talked about overcasting edges to avoid fraying. Overcasting isn’t just important in treating the edges of your finished project – you may also want to do it before a prewash. You’ll either spend the time overcasting before washing, or detangling after. Treating the edge before washing leaves the fabric easier to work with when you’re ready to get sewing.