I'm thinking just using a back screen protector.
You mean a screen protector on the front glass? That's all you need, unless you like to drop phones on the cement/pavement, then you might want a case, just in case.
I have zagg invisible shield HD front/back of my black iPhone 5, but, I probably could have gotten away with just the front screen protector. There scratches in the tiny beveled edge, but, it's not a big deal, it doesn't scratch easy all over like some people were claiming when it first sold.
I still luv my iPhone 5, runs iOS 7 beta well, not upgrading this year. There's no hurry though, I think $99 will be the regular price for 16GB iPhone 5 here in a couple weeks. (I boycott walmart myself).
This is my guess for new iPhone lineup:
iPhone 5C 16GB only $50 on contract? 6 Colors
iPhone 5 32/64 GB only $99/199 on contract? White/Black
iPhone 5S 32/64/128 GB $199/299/399 on contract? Graphite/Champagne/White (fingerprint sensor here only)
OR
the 5C might be 8 GB only, iPhone 5 starting at 16 GB