old's'cool

Unlike the frame and most of the rest of the bike, there is another property of wheels that matters in addition to mass, which is moment of inertia, as someone mentioned above. Because moment of inertia is partially a function of mass, that leads to the various axioms about the relative importance of wheel mass to overall mass. In fact, considering only steady speed riding, for a moment, whether on a flat, or uphill, or downhill, the mass of the wheels is no more or less important than the mass of the rest of the bike.
However, when accelerating or decelerating, the wheels' moment of inertia becomes important. Because of moment of inertia, when accelerating or decelerating, an ounce of mass in the rim or tire requires as much effort to accelerate or decelerate as 4 ounces of mass in a non-rotating component. An ounce in the spokes, on the other hand, since their mass is distributed from the hub to the rim, is equivalent to about 2 ounces in an non-rotating component, again, only when accelerating or decelerating. And the hubs, since their mass is concentrated near the centre, make a minimal contribution to the moment of inertia, and might as well be considered non-rotating, as far as their effect on acceleration and deceleration are concerned.