I do not prewash because I love the feel of the new fabric as I'm working with it. I will test any dark colors that look suspicious (reds & greens). I've never had a problem with the raveling, but I know some do. If I should prewash for any reason, I always clip a small snip from the raw edge corners, helps with any raveling during washing, plus if it gets put back in my stash, I will know that it has been washed already. Many of ladies in my quilting classes do this also...I'm sure it has been done for eons! ;-)