In Pennsylvania, drivers must report a traffic accident to the police anytime someone gets injured, even if it is a fender bender. You are also required to report a crash if either vehicle requires a tow because it cannot be driven and moved on its own. You should also remain at the scene and report any hit-and-run crash you are involved in as a victim.
Reporting minor incidents, such as a fender bender where the damage seems minor, may still be a good idea. The reason for this is that sometimes people who claim they are not injured may change their minds later. If you have a police report, you have an official document of what occurred.