Most people use a blanket stitch or a satin stitch to hold down the edge of machine applique. If it's a wallhanging, you can also do a regular straight stitch edge about 1/8 in. in from all your pieces for a raw edge applique approach. It's your wallhanging so do whatever stitch that you like. Try out a few on a scrap until you find one you like. If your pieces have some kind of fusible on the back, they just need a little extra stitching to keep the edges from coming up when the piece is washed.