I usually do the outline stitching on the quilt top before quilting especially if I have a lot of thread changes. Since your piece is just a wallhanging it would depend on what you are going to it. Are you going to outline stitch the figures or put quilting inside them? If you are going to kind of thread paint all the details, I think I would do it while quilting. If you are just going to do the main outline on the applique I think I would do it with invisible thread so that you wouldn't see the colour changes over all the different colours along the edge. Beautiful block by the way!