Seems to me that there's only two reasons to copyright a song: a) So that if someone else wants to record or perform it, you can get a piece of the action and b) So that if YOU record or perform it,nobody will be able to make a sucussful claim that it's his song.

b) is self-evident. All you need for that protection is to mail yourself a copy of the song (certified mail) and not open the envelope until you're challenged.

The same protection would work for a), but there's less of a chance that Willie Nelson will stumble across the song and earn you thousands of bucks recording it. If you want exposure, try posting the song (WITH copyright info) to the DT; people looking for this type of music are much more likely to find it here than at, say,ASCAP or BMI