For a wall hanging I would NOT wash it as I like smooth, crisp looking fabric art, not the crinkly cuddly look of a quilt. Here is how I make a hanging sleeve:
i use muslin. Cut your hanging sleeve piece 9" wide and the length of the top of your quilt plus one inch. Fold in both short sides 1/2" then another 1/2" and sew next to the edge making a finished hem on both sides. Fold the long sides together (wrong sides together) and sew with a 1/2" seam allowance, making a tube. Press the seam allowance open. Then press the long tube flat with the open seam allowance centered on the back side of the tube. The seam will not show since it is placed directly on the back of the quilt. Place the tube 1" below the top edge of the back of the quilt with the tube seam next to the quilt. Fold the top of the tube down 1/2" and press. This will create extra fabric for the hanging rod so that it won't 'draw back' the top of the quilt when the rod runs through the sleeve. Hand stitch the long sides of the tube to the back of the quilt being careful to just catch the tube, backing and batting in your stitches.
I have eave read some people put the hanging sleeve in the binding seam, but I have never done that---seems like it would make the binding hard to sew on and would be too close to the top of the quilt and might show when hung.