Copyright
Sending a copy of a song or poetry to oneself is not a legally recognized form of copyright. It's up to the judge in a case like this, and if he decides to side with the man who stole your tune (who may have registered it with the Library Of Congress) then you've lost your song. Put yourself in the judges position, all he sees is two people who are both claiming ownership, one has spent $3.00 to register the song, the other did not. If he's a stickler for details, (and the letter of the law) he doesn't have to acknowledge the fact that your self-addressed-letter predates your opponents copyright date. It may not be fair, but honestly, that IS the way it is.
ALSO, I wouldn't trust my HIT song to a lawyer NOT versed in MUSIC LAW,(One familliar with "Intellectual Property").
It has already been mentioned, but the most efficient and affordable method is to wait until you have 8 or 10 songs finished, then register them as a 'collection'. Use form 'PA' and it's only $30.. for the whole bunch. Later, if a publisher is interested in one (or more) of those tunes, you can re-register that song by itself just to be sure that everyone sees it on the L.O.C's web site as your song.