I always prewash and I don't do anything to my fabric before washing. It has been my experience that the higher quality fabrics fray very little. Low quality fabs fray more.
Flannels and homespuns will fray a little more than most. I have never lost more than 1/4" of fabric to fraying and rarely do I have that much fray on me. I lose more to inaccurate cutting on the part of the seller and squaring up. Too bad fabric shops don't give us an extra 1/2 to 1". I am probably dating myself but I can remember the days where that was the norm, to automatically cut the customer an extra inch to account for the fabric being out of square on the bolt. :?