I had an absolutely perfect iPhone until two bubbles appeared on my screen in October. I debated whether it was worth exchanging the phone over. A month later a third bubble appeared. I decided to go ahead and exchange it. I had bought my phone in June without Applecare. I made an appointment at the Genius Bar, brought nothing in except my phone, and they exchanged it, no problem.
Except, that the second phone they gave me had a defective battery and wouldn't hold a standby charge for more than 8hrs. So, I made another appointment and they exchanged it again, no problem.
Except, the third phone still has somewhat of a battery issue. It never charges fully and I can only get the usage to reset if I charge it with the wall charger. I'm tired of exchanging it, so I'm living with it for now.
So, the moral of the story is only exchange your phone due to cosmetic imperfections if you feel you can accept the risk that you may end up with a defective phone or battery in the process and that you may have to exchange your phone a couple of times until you're satisfied or until you give up trying.