Apple promises Security and Privacy, this is now enforced with Legal Protection, requiring a warrant after the initial search. At least this is the opinion of a Seattle District Judge, as made in a recent ruling. Judge Coughenour clearly states that the Fourth Amendment protects your lock screen from being viewed, or photographed, against your wishes.
The FBI do have the right to view lock screens at the actual time of any lawful arrest. The judge, though, went further to explain that “any future examination of a lock screen would require a court-issued warrant”. This means that lock screens are protected soon after a lawful arrest, even during the federal inventorying of confiscated personal effects. Moreover, only the agents making the lawful arrest are empowered to view the lock screen, and then only at that specific time.
Powers of Search only valid at the time of arrest.
This fine point came to light because the lockscreen in question had an, as yet, unaffiliated alternative identity/nickname prominently displayed. This identity definitively linked the arrested individual to a known name the agents had been generally trying to locate.
Confusingly, this means that although a passcode CANNOT, and a TouchID unlocking CAN be compelled by law enforcement; the actual viewing of the said phone lock screen can ONLY be viewed AT the time of arrest, BY the agents making the arrest.
Know your rights, always get professional advice. In the meantime: choose a jazzy lock screen, that your momma will be proud of!