Note that if you use more racks than RF you lose some of the operational benefit. e.g: you'll still only be able to take out one rack at a time (especially if using vnodes), despite the fact that you have more racks than RF. As Jeff said this may be desirable, but really it comes down to what your physical failure domains are and how/if you plan to scale.
As Jeff said, as long as you don't start with # racks < RF you should be fine.
