I don't think Rin's teeth are intended to like, "actually" be that sharp, I think it's just a stylistic thing since they associate him with a shark and all. So I mean, if you were writing fic that was trying to approximate reality more, obvsly a human being wouldn't have sharp teeth like that anyway so he'd just have normal teeth. I'd only make them weird inhuman shark teeth if the fic were about someone getting their dick bit off or something. (Ahh, my solitary swimming anime kink meme fill!)