I had this EXACT issue on my iPhone 5. It really bugged me for a week or two. I was very irritated by it and actually was planning to exchange it when the crowds died down.
After those first couple of weeks though, I have honestly completely forgotten about it until now when I saw your picture, and I had that phone for two years and over a complete year of that was case-free.
So in hindsight I would say it's not that big of a deal.
Steve Job's ghost is very angry... expect the Job's mansion to report the sound of footsteps, slamming doors, and flickering lights tonight. His spirit yearns in pain because of this poor build quality.
If this was a Samsung phone, people would be all over it saying Apple would never let this happen.
Honestly, just put a damn case on it before it breaks anyway. Who cares as these devices were designed assuming you would be putting a case on it, I mean look at the camera lens.. the phone looks like the profile of Pinnochio.
If everything else is perfect, ie no screen issues or loose buttons, then I would keep it for sure. You never know what issues your replacement(s) might have and crooked text on the back is nothing compared to the faults you could end up having to deal with.
Same as me basically, much worse in person than in pics. I have a clear case on it and must say it bothers me. It just cheapens the look of the phone.
For the exception of a tiny scratch in the stem of the apple on back, and of course the slanted text on the rear of the phone I have no other issues, which honestly gives me a stomach ache because I know how this whole replacmemt game goes with apple.
You replace one iphone with an issue, to only end up with another one that has a dead pixel, faulty home button etc. I'm going to wait a week and see how I feel then.
Some of us are still waiting for our phones to arrive after preordering early, so I do envy you having your phone in hand. With light leakage and dead pixels and dust under the screen and battery glitches and bending and yellow screens to be on the lookout for, I'd be thrilled if the only flaw on my iPhone is some crooked little text nobody will really ever notice unless they're looking for it. I myself will never see it even if I go without a case because I'll be too busy ogling that spectacular big screen. Don't get me wrong, I'm OCD about a lot of things; I understand why it bothers you and understand you feel it makes the phone look cheap. It's just that it seems really wasteful and risky to me to return an otherwise perfectly functioning and nicely crafted phone over something so hard to notice for most people.
Edit: I just noticed your screen name. I bet you never get around to returning it anyway!