I soak each dark fabric individually, no exceptions, in hot water. If it bleeds, it gets pre-washed in warm water with other bleeders, using 2 Color Catchers.
I do the same thing with other fabrics, too, but use just 1 Color Catcher.
My reason: low trust level! I wouldn't want to put in all that hard work to watch colors run later.
That being said, my LQS owner says she never prewashed quilt shop quality fabric, but puts 2 Color Catchers in the wash when she washes the finished quilt. I do not have that kind of trust in fabric!