You are never obliged to report crimes to the police unless you're a mandatory reporter (basically, doctors, social workers and teachers) and the crime is on the mandatory reporting list (child abuse, and that's about it). If you're not a mandatory reporter, then you don't have to report anything for any reason ever. It's actually constitutional, by the way. It's part of the fifth amendment. Basically, the fifth amendment means you can always keep your mouth shut in case you accidentally say something incriminating. In fact, the mandatory reporting requirement has never been tested, and I think is likely unconstitutional.
Additionally, your understanding of accessory is a bit off. To be an accessory, you must know about the crime and provide material aid toward the commission of that crime and there has to be an accessory statute for that crime. For example, if you give a ride to a dude who then commits a murder but you didn't know about the murder, you're fine. If you pick him up afterwards, find out about the murder, and hide him, you're now an accessory after the fact. Another example, you stand in front of a camera so your friend can shoplift, you're not an accessory. Why? Because there's not a statute making being an accessory to shoplifting a crime (in most states; some states have a more general accessory to a crime law which would cover it). You may, however, be guilty of something else like conspiracy to commit fraud, or may even be considered a shoplifter under some statutes, but not an accessory. Final example, you find out about a friend committing a robbery and don't report it to the cops. You're not an accessory, since there's no material aid.