I never prewash my flannel for my rag quilts and I have made about 20 now. I make them for every new baby and some of my older babies who use them on the sofa while watching tv.
I think it is important to wash them after rag cutting the edges and I usually wash them 3 or 4 times before giving them away. I also clean out the lint filter in the dryer about every 10 minutes because I was advised that it will have a lot of lint. It does. I use 8 inch squares and some rectangles. I have never done a triangle.....good luck and have fun with it.