I actually had to exchange my iPhone because the screen had yellow spots on it. The Apple Store requested that I brought back the box and all original contents. You don't need the receipt. They can look up purchase info by the serial number.
Call me wrong, but I did take out the Apple stickers and the cleaning cloth before exchanging my phone. They don't check for that. I have a stack of Apple stickers from every Apple product I have bought and I figured I could use the extra cleaning cloth