During the PAD Challenge, I noticed quite a few poets including either the word Copyright or the copyright symbol--a C inside a circle. While I understand the fear of someone stealing your work and may have even done that with my own fiction and poetry earlier on as a writer, I want you to know you don't need to include those markings, especially when you're submitting your poetry to journals and magazines to be published.
Reason #1: People don't tend to steal other people's poems. It's just not profitable AND if someone were so inclined, they would steal the poem whether you include the symbol or not. Once you set your writing down in fixed form, it is protected by copyright. But after more than 8 years working on Writer's Market, I have yet to hear of a case where an unknown poet has to take his or her poetry copyright case to court. (Of course, saying that, I do realize that there's a first for everything. For more info on copyright, go to http://www.copyright.gov/).
Reason #2: Adding the copyright symbol does not increase your chances of getting published. There is no editor who sees the copyright symbol attached and thinks, "Yay! We've got a copyright symbol; let's get this issue out now!" In fact, it often hurts your chances, because...
Reason #3: Adding the copyright symbol to your submission marks you as an amateur and as a poet who is paranoid that the editor will steal your work. While an editor would still accept exceptional work from a poet who includes the word Copyright or the copyright symbol, be aware that those markings will distract most editors from reading your work--even if just the tiniest bit.
So that's my practical advice about including the copyright symbol and/or the word Copyright. It doesn't decrease your chances of having your work stolen, but it does increase the chance your work won't be accepted. So, why do it?