WHEN SHOULD A SONGWRITER FILE FOR COPYRIGHT PROTECTION?
The writer doesn't actually need to register a work at the U.S. Copyright Office in the Library Of Congress ( http://www.copyright.gov/ ) in order to own a copyright. Registering the work(s) makes proving copyright ownership much easier in a court of law when there are disputes. If you are concerned about someone ripping off your material, by all means copyright as soon as you feel your work is finished.
A copyright notice should be written as follows in order to claim a work as yours: "© (Date and/or year)(Your Name) (All Rights Reserved)." For phonorecords of sound recordings the "C" in a circle is replaced with a "P" in a circle.
A "Poor Man's" copyright ( mailing your material to yourself in an envelope which you keep as dated proof) is not going to hold up in court, so I would advise against doing this. The bottom line is, copyrighting through the U.S. Copyright office affords you the best means of proof.
Whether you have just one lyric or song, or a collection, you can copyright individually or as a "Collection Of Works" for the same $45.00 fee. Copyrighting individually, however, affords the artist more protection than copyrighting material as a part of a collection. Click on the link above to visit the Library Of Congress' information base to discover the differences in copyright protection.
For practical purposes, a songwriter should only file for copyright protection after final production of the demo, when you are satisfied completely that the song is finished--not before changes and tweaks are made necessitating copyright amendments.
Songs are rarely stolen; what may happen more often is that a song idea may be stolen, and a new--possibly better song--will be written based upon your idea. This is not uncommon, and industry pros are known to exploit great song ideas. That's their bread and butter.
Unfortunately, song ideas are not copyrightable, so keep your project as quiet as possible until you've recorded the song, and then, by all means--register your copyright!