The 'poor man's' copyright method of mailing it to yourself so you can establish a date works, really, only from the standpoint of establshing an origin date. But that's completely easy to fake.
If your song was to be stolen, the only thing you could do is to take the thief to court. Most likely, you wouldn't be taking them to court unless this person or record company was making money from your song. If they're making money from your song, then this means they're going to have an expensive lawyer on their side. So you'd better have one, too, or you're going to get bamboozled.
When the judge asks to prove that you wrote the song, it becomes a complicated thing and is always a case-by-case basis depending on all factors involved. So a person steals your song. You'd better be ready to proove it.
It's too easy to fake the poor man's copyright, and the courts know it. Plus, it's much better to get a copyright registration from the LOC because this means that the gov't will go to bat for you and will proove to the judge that you are the original creator. The other advantages are that the gov't then guarantees that you will be reimbursed for any legal fees when you win your case. Not to mention the statutory damage award.
$45 (today's fee) is a small price to pay.
Check out this webpage.
Steve