Not exactly sure what the question is! Probably because of terminology? Knifey rails to me means thin rails like on a hull, where the bottom curves up and the dome of the deck curves down, and the rail itself is thin where they meet, with not much meat to round off.
Hard down rails in the tail are pretty common around here, and if you like a square edge for release, then you're best served by damming the resin during the hot or gloss coat, especially if you wrap your rails, since fiberglass cloth does not like to wrap around a 90 degree angle. I keep a small radius along the rails in back while shaping, then dam the resin later.
While shaping, I shape the top half of the rails from above, and the bottom half from below. I just remember to barely touch the rails in the tail when the board is bottom up, and gradually work the rails more as I near the half point and forward. If I'm going for a "beak" nose, then I back off again at the very tip. Finally, I put the board on edge and clean everything up at the last.
I dunno if this answers your question, or helps, but hey I tried!