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,.
In general I try to follow the pattern directions for casting off but if it’s not convenient or it doesn’t suit my mood, I don’t. For a ribbed collar, where you cast off in patttern, I really don’t see that it matters.
Do you mean Cast OFF? You can do it on either side. I like to use a purl cast off on a RS row with a stockinette project to use the purl ridge as decoration and a slight counteraction for stockinette’s rolling tendencies. And sometimes I do a purl or knit one on the WS. Just whatever you feel like you want.