Here is a close up of the sleeve seam on the inside (on the left) and the outside (right). I apologize for the quality- I just laid the jacket on the scanner.
The bottom pic is of how the finished jacket hung. I finished the non-cast on edges with a single crochet. I have to tell you that the continuity of the seams and the edges just makes me happy.
Having said all this, I also get a huge kick out of doing invisible side and sleeve seams using the mattress stitch. I appreciate the tips above on how to keep the stitches lined up. Up until now I have obsessively counted and marked both sides with colored straight pins. I am excited to borrow my DDs’ hair clips for my next project.