I'd always wondered about the (relatively) small fins on the Saturn V (they are fixed fins BTW, not actively controlled). However when I read a book on the development of the Saturn rockets (I, IB, and V), it talked about those small fins on the S-V. While obviously of little use at low speeds, the static aerodynamic stability they provided at speed dramatically reduced the amount of nozzle gimballing required to keep the rocket on-course.