The prohibition against coiling the cable comes from the world of radio and TV. Broadcast signals can end up causing ghosting if the cables are coiled up.
You can coil Cat6 to your heart's content, just don't kink it or bend it too sharply.
BTW, lots of times Cat6 gets specified because that's "the IN thing to have". If the network can't handle those speeds, the full capabilities of Cat 6 are never touched.
