It depends upon which way you rock the bike. What Compressed mentioned is when you rock the bike away from the power-stroke leg. So if your right-leg is on it's downstroke, you rock the bike to the left. This requires extremely strong arms and back to counteract the leg-muscles. This effectively raises your gearing as it artificially shortens your crankarm-length. I think this is the only scenario where rocking the bike helps.
If however, your bike is rocking towards the power-leg, that is, it rocks to the right as you're stepping down on your right-leg, then some of the leg's power is going into rocking the bike instead of driving it forward. You could probably pull up on the bars a little more in this case.