I vote case...
Regardless of how it looks, you should get a case to protect it from drops, dings, scratches, etc. they are pretty sensitive. My cousin decided to go "naked" with his iPhone and just recently dropped it from less that a foot distance. It completely shattered... So to case is smart. Not to case is not too smart.
A bulky phone looks better than a broken, scratched, dinged, shattered phone any day!
I recommend otterbox. I have the defender series which is perfect if you drop or are afraid of dropping your phone. Well worth the price!
Beats having a broke phone. Then having to replace said phone after it breaks. Replacement parts cost the same or more than cases. I'd just go with a case.
Nobody can answer this question except for you. We can only say what we would do. Lots of variables at play, as for me, I always have my phone in a case before I leave the store where I purchased it.
Works for me, YMMV. I will also recommend that if you decide not to case you should make sure you have it covered under some type of insurance plan. These things are expensive to replace.
Right now i am choosing to go naked, except for Zagg front and back invisible shields (dry). From day one in october i have had my iphone 4s in a case. since then i have bought many cases, but not happy with any of them. Seems to be just something i don't like with any of them. The otterbox commuter is a good case for protection, How ever i think its one of the ugliest cases out in the market, and puts more bulk to the phone then i would like. Other cases interfered with my screen protectors, or i simply just didn't like them. I have had cell phones for over 20 years now and rarely have i ever dropped my phone. At work i do keep my phone in a belt pouch and i wear a Bose Bluetooth Series 2, I can access siri and make calls, and text without even taking my phone out of the pouch. And Ever since i decided to go naked i like the phone alot more then with some cheezy case on it.
I am using the mophie juice pack plus. Gives you some protection and the ability to charge your battery up. When battery hits 10%, I just flip switch on case and it charges phone back to 100%. I would suggest a screen protector for front.