Personally, it's not necessary. Unless you're willing to use your hard earned money to go fight for compensation in the court of law. It's a costly exercise. Legal fees will eat up most of whatever money you make. In the end, the lawyers win. So unless you're willing to spend big on legal fees, there's really no point. But, hey, that's just my take on it. Don't flame me. Just an opinion.
It's quite common to post your lyrics in by registered post along with a CD to yourself. Then keep it sealed for future evidence.