Let me share my experience.
1. My first iPhone 5 (#1) had a speck of dust under the glass.
2. Replacement phone (#2) had a pretty bad scratch at the bottom left bezel down to the metal.
3. Took phone #2 into an Apple store for an exchange and looked at phone #3 which had even more scratches.
4. Came home with phone #4 which was cosmetically flawless. Noticed a dead pixel in the middle of the screen.
5. Back to Apple store and looked at phone #5 which had some scratches.
6. Took #6 home which was flawless cosmetically but had a speck of dust under the screen.
7. I am keeping #7 which is perfect cosmetically without any dead or stuck pixels but has a couple of very fine micro scratches on the glass which can be only seen at certain angle under very bright light from a single source.
Moral of the story is that the perfect iPhone 5s in black color are extremely rare.
If you suffer from an OCD, do not purchase a black iPhone 5. You will certainly need a therapy after going through multiple exchanges.