I prewash and dry everything that I'm going to use in quilts that I know will be washed and dried along the line. I've had fabrics bleed that I didn't expect to--light oranges, yellows--and certainly purples and blues as well as reds. Oftentimes when I've put the pieces in to soak it doesn't look like they're going to bleed until I squeeze them out and then huge amounts of dye are released. I'm talking about even 5-inch squares.
My feeling is: I like to start out putting my quilts together being pretty sure that nothing is going to bleed into its neighbors and that none of my pieces are going to shrink differently than others when they are eventually washed and dried. Quilting is too much work to waste the effort on something that I might not like the results of down the line. This is strictly IMHO, of course.