I don't prewash and have never had any problems. Only one quilt, I know of faded when it was washed after making it. I don't know how the person washed it or how long it was left wet before it was put in the dryer. I do know that when I saw the quilt not only had it faded, but it looked like a limp rag. The color that faded was a dark green and it was only in some areas where the quilting was close together.
I recently made a lone star out of dark and bright batiks along with regular cottons in the same colors. It had a White on cream backing. I washed after making and it came out just fine. I've also made at least three quilts using reds, blacks and white without prewashing. They all turned out ok being washed after finishing.