When I first began quilting, I was told to prewash the fabric before cutting. Later, the people at my LQS said that they did not. I came across some fabric that I had prewashed, and I would like to buy some complementary fabric to use with it. Should I prewash the new fabric, because the other has been washed already? If I mixed prewashed and unwashed fabrics, will it cause problems when the whole quilt is washed?