depends on where the light is placed near the wheel. I am pretty sure my lights are forward enough there is no shadow, or it's right at the edge of the light beam. I see on some of the racks in this thread that the lights are mounted further back. I know that when I have lights on the fork crown, the shadow occasionally annoys me.
For a bike with a metal fender, there are fender mounts. Velo Lumino is the easiest one to buy. I have one of those, but I haven't used it yet. I think that might be the best place to put a light.