Copyright in qualifying United States works has always arisen upon creation. For many years, however, rights could be lost by failing to, e.g., provide notice, register and deposit copies when works were first published. In 1909 formal requirements were reduced, and the Supreme Court, in 1939, concluded that registration with the U.S Copyright Office was unnecessary to retain rights. Despite that, owners could not sue infringers without having registered.
Regarding registration as helpful if not obligatory, this paper compares the burden on moving parties in circumstances in which registration decisions may be challenged in courts.
IDEA: The Journal of Law and Technology
Thomas G. Field, Jr., "Judicial Review of Copyright Examination," 44 IDEA 479 (2004).