After sewing in a sleeve and checking to be sure you haven't sewn in any tucks or pleats, you can reinforce the seam by stitching again about 1/4" away from the seam in the seam allowance.  If this is a 5/8" seam allowance, you should then trim away the excess seam allowance beyond the second stitching line between the notches and the side seam edges (making those ends of the seam about 3/8" seam allowance).  This eliminates the need to clip the underarm curve.