As one Apple retail tech told BI,
General Damages and Issues
Under warranty, Apple will cover debris under the screen, camera foam misalignment, and a single hairline crack with no evidence that a drop or hit made that crack. You'll have to pay for cracks inflicted by drops or hits, water damage, and other obvious user-error damages. If you've tampered with your iPhone in a weird way—i.e. used non-Apple parts or run it over with a dump truck—there's no way you're getting a discount on repairs.