There are a lot of parts to bicycles that can benefit from 3D printing. Seat, hand grips, lugs... I think that the wheels are probably the least likely to benefit from 3D printing. Spokes are the reason. The wheel is held together with tension. I'm not sure how you would print something with tension built in. Though I guess you could print it and tighten it up. Printing a wheel with a tire would be even more difficult because of the variety of materials required.

But 3D printing currently works best with custom objects, objects that can't be built any other way or for prototyping objects to be mass produced later.