There's never a shortage of discussion on this topic
.
I prewash everything for the following reasons:
-peace of mind (don't want any disappointments from bleeding or shrinkage once the quilt is made or washed)
-cleanliness (some warehouses and shipping containers have bugs)
-more time to pet my fabric
-more likely to remember what I bought than if I bought it and stored it away till ready to use
The best prep I found for peace of mind is this recipe from the Amish, given to me by LQS owner (who admits to only using this method on really dark colors because she is too busy making quilts on her days off).
http://www.quiltingboard.com/links-r...g-t237701.html
That said, I preSOAK each new piece of fabric in the bathroom sink. Sometimes I'm surprised at which colors run. If a color keeps running after a few soaks, I don't use it.