Shaping seam allowance corners really helps when matching the pattern pieces up and it's not difficult to do. Since I add seam allowance when I trace the pattern to the fabric I do the shaping then. First I add 1/2 inch seam allowance.
Then I flip the pattern piece over and trace off the correct angle.
When it's time to assemble the pieces there's no question as to how the pieces should line up, and on seams where the allowances are pressed out they perfectly match the rest of the piece.
Sometimes it's difficult to be sure exactly where seam lines should match up. Shaped seam allowances solves this issue. Since every fraction of an inch can make a difference on a corset I like to be as precise as possible.