I prewash, to get all sizing and extra dyes out. If I think a dark color is going to run, I wash it separately (in the washer with regular detergent) and then rinse with salt and cold water. It helps to lock in the colors or so I learned in Home Economics class in, oh, a couple of years ago just before I graduated high school. ;-)
Sharon
PS, lucky catch on that dark streaking fabric. Whew, I'd have been really "ticked" to have invested in quilt only to have it fade and streak!