I always prewash too.
Like many others, it's not worth the risk to me. I prefer a smoother look to my quilts so don't want the shrinkage, not worth the risk of bleeding and I don't like working with fabric with all the chemicals still in it and many other things.
There's likely more but the end is I prewash everything. Others don't but I do and likely always will unless someone can show me how prewashing would do damage! LOL