yes 90% of the time its the only thing between the outer and inner fabric.
if i'm working with silk, other difficult fabrics, or i really want the bag to hold an odd shape - i'll use a product called Fuse A Shade.
its a thin but firm non-woven fusible product. i'll put that on first and then i fuse the Pellon Fusible Fleece over it.
i also put the pellon fleece in the straps of the bags.