I've noticed that patterns specify (sometimes at least) to finish with a WS row and then CO on the RS. Casting off on the knit side of stockinette is easier and faster but does it matter, really? Right now I'm finishing a ribbed collar and must decide whether to work another WS row and cast off on the RS. I don't think it matters much which side I'm on. The row I CO will be the edge of the collar, not a seam edge,.