If you use the two sided fusible (Heat&Bond), the edges of the applique will eventually start fraying on the edges even if you use a buttonhole stitch. A tight zig zag will prevent this from occurring.
When you sew the light fusible to an applique and turn it inside out, you have turned the raw edges to avoid the fraying. She started using this technique I think when her Sunbonnet Sue book came out and continues using it.
I did see someone use washaway interfacing doing the same technique of sewing the edges and turning it inside out and pressing. When the applique has been pressed into place and starched, she mist the wash away with water and then you just have the applique without the bulk of the fusible interfacing. Many techniques to choose from. Use the one that works for you.