I would just test the yardage for colorfastness. If it is colorfast, I would wait and just wash the whole quilt.
Also, it does not hurt to mix washed and unwashed fabrics as long as you are quilting reasonably close (every 4 inches or so). Quilting will keep fabrics from shrinking more than the batting shrinks. The problems with distortion typically come from mixing washed and unwashed fabrics in a tied quilt; the ties are not sufficient to prevent the fabrics from shrinking unevenly.