I don't usually prewash, but if you are worried about a fabric possibly bleeding, then definitely wash them all. Flannel does ravel and shrink pretty bad. I always zig-zag (or serge if you have a machine) the raw edges before I throw them in the washer. Saves a lot of time later untangling the fabric and cutting all the threads off!!
And throw a Shout Color Catcher in the washer with the fabrics.