I always prewash. The first quilt my DIL made was flannel, when she washed it all the seams pulled apart and it was ruined. I wash flannel in warm water and use 3/8 to 1/2" seams. One of my studens washed a finished quilt that she had made for a friend to sell. The colors ran and almost reuined the quilt. She was able to get it to a softer color and the lady paying was happy with it. It is a very good idea to give color cathers with every quilt you give way.