It'll matter if a fabric bleeds onto something else and you can't get it out. Other than that, I can't think of a reason to prewash fabric for a show quilt.
Even if a fabric bled it wouldn't necessarily disqualify you or cause you to miss out on a ribbon - it's all in how you handle it. I know of one machine quilter whose quilt became blotchy. She tea dyed it, and it looked fabulous.
Janet