I know that the prewash issue has been discussed to death. However, I wanted to share my recent experience. I'd purchased two bolts of fabric - red and muslin. Since I'm starting my Dear Jane journey, I don't want any shrinkage issues at the end. I washed both the red and the muslin in the hottest water that I could. The muslin shrank 8% in width and 11% in length. The red shrank 11% in width. I didn't measure the length - it's on the bolt and I don't want to unwind again
These numbers are why I always prewash. Different fabrics shrink different amounts in width and length. If I'm doing applique, for example, I don't want that applique to go wonky... if I want a circle, I want it to remain a circle. This is just my view. Each to her/his own.