Bonnie Hunter (
www.quiltville.com) has talked about using fabrics that are washed and unwashed together. She is one of the most prolific quilters that I've ever seen. She's never had a problem.
I don't prewash - I prefer the crinkly look - and have not had a problem with anything yet. If I'm using a fabric that I'm concerned about - reds, purples - I test a piece.