Our guild did a redwork block of the month and it was suggested to use a backing to hide traveling threads as mentioned above.
I had some lightweight woven cotton fusible ($6 range @ Joannes) from another project, liked it because it has a straight grain like regular fabric, so I used it on my blocks and liked the effect alot.
Some others used Thermore; some used thin white cotton batting from Quilters dream; others used muslin. Flannel has a shrink rate that might keep me from using it for this kind of project.
Jan in VA