Yes, you will need to add seam allowances and how much would depend on how you are going to do the channels for the boning. However, on a toile, I would leave more than the seam allowance so that you have room to manoevre (how is that spelt?!?!?!) during fitting and you need to let out any seams, depending on how tight you are intending to lace the corset and to ensure that the back lacing panels are kept straight.