I am not an expert by any means, especially with machine applique but it looks to me that the edges of the applique fabric is very well covered by the stitches you are using. This will keep the fabric edges from fraying and coming lose from the background fabric. The applique I've done has been with fusible web and then I do a machine blanket stitch or just a straight stitch near the edge. Those stitches would not prevent fraying on their own and the fusible helps. So I think yours will work just fine and your blocks are cute as can be!