The only thing I try to think about is not to slide it across surfaces in fear of scratching the lens...
I used to think this as well, but since I figured out it's a sapphire lens and can't be scratched by almost anything, I decided to go naked. After having to replace two iPhone 4 back covers because the lens got so scratched (could no longer take clear pictures), I didn't want this happening with my 5. Basically the only reason I had a case, but I'm very happy for the new camera lens material!