I do know musicians who call this "poor man's copyright" and swear by it. If they write a song, words and music, they send theirselves a copy by registered mail and don't open it and they say this potects them if someone else claims or uses their words and music. Can't swear by this but it does make sense, as copyrighting something is much easier and cheaper than getting a patent.
My friend does this too for her artwork, before she posts it up anywhere she makes a copy and mails it to herself. The PO puts the date on it, and if it's unopened it's the earliest recording of it.
She's never used it in court, but it does work for run of the mill art thieves so far.