Machine Stitch Exposed Bound Armhole: Invisible Stitch Finish
If you are not a big fan of hand stitching and do not particularly enjoy the casual look of a topstitched binding application, there is a clever technique you can use to seal a binding application by machine, with minimal stitch visibility on the garment's face side.
Once the binding has been stitched, it is folded over the garment's raw edge to enclose it for a clean finish. During this process, instead of aligning the binding's open fold with the stitch (visible on the wrong side of the application), fold the binding a little past the stitch to fully encase. The binding's open fold should be aligned about 1/8" past the stitching. Hand baste the binding in this position to secure this alignment.
To permanently seal the binding, topstitch directly in the crease or seamline formed by the binding and garment on its face side. This stitch should catch the binding edge underneath.
In the finished application, the machine stitch should blend into the binding-garment crease on its face side, and be fully visible on the garment's wrong side as a topstitch applied directly along the binding edge.