I always clip the 4 corners of my fabric before I prewash...then between the washer and dryer, I pull off/clip off any threads that are loose. I don't know why it helps to clip the corners, but it does. The only fabrics I prewash anymore are flannels, as they have a higher rate of shrinkage.
I do have a friend who always serges the edges of her fabrics before prewashing-so that is another option.