There is no such thing as a "poor man's copyright." It does not exist in law. Under the current copyright law, you own the copyright in the work from the moment it is fixed in a tangible medium (sung onto a tape or CD, written down on a piece of paper, etc.) Make sure you have it fixed in a tangible medium before posting. While you could use the posting date in a lawsuit as some evidence of a prior date that you had the work to which someone else is claiming ownership, registration is prima facie proof of ownership - which means if someone else has registered it before you, you have an uphill battle in court because the Court must consider the registered owner to be the owner in fact (until you can convince the Court otherwise by sufficient proof). In addition, as stated above, you must register the work before you can take legal action, which means that you forfeit any statutory damages to which you would be entitled if the infringement occurs before the registration. The bottom line is that you should, at the very least, send in for the registration before you post.