The Ares I rocket doesn't have fins. Instead it relies on gimballed engines to control the direction of thrust because any fins would have to be ridiculously huge in order to stick far enough outside of the rocket's boundary layer to have any effect. I heard a rumor that the fins on the Saturn V rocket are actually useless for the same reason, but von Braun ordered their addition anyway because it just didn't look right.
Is there any truth to that?
No, Werner von Braun was not an idiot.
The fins on the Saturn V were not useless. They did not provide directional control (that was done by gimbaling the engines.) Their purpose was to increase stability in the event of an autopilot or engine gimbal failure so that astronauts could safely abort.
Source: 1964 Popular Science interview with von Braun
Check out page 185 for the most relevant part.
the fins reduce the aerodynamic instability enough to make sure that the astronauts can safely abort
Our aim is to reduce the "turning rate" - the rotational speed at which the aerodynamically unstable Saturn V, when stricken by an autopilot failure, would turn into an angle of attack at which its structure would be bound to fail. One might say the purpose of the fins is to extend the period of grace that the astronauts have to push the "panic button."