Is it necessary to copyright your work?

A few days ago amz left a comment on one of my posts regarding the copyright I have in my header. She was wondering if she should copyright her poetry before posting it on her blog. So as I was surfing the net, I came across this article from www.writersdigest.com:

Q&A: Copyright Before Submission?
Q. Do I have to copyright my work first, even if it's a short story,(poem, etc.) before I send it off for someone to publish?
--Philip Atkins

A. Your work is protected under copyright law the second you write it or type it (as soon as it is "fixed in a tangible medium"). That said, registering your copyright with the Copyright Office of the Library of Congress will help you prove your copyright in a court of law. For more information about registering your copyright, visit here. Keep in mind, however, that a publisher will re-register your copyright once your book is published if you are writing a book.
If you do register your copyright before submitting your work, we don't recommend indicating that on your manuscript. Some agents and editors feel that including the copyright information on your work is a sign that you don’t trust them, that you’re warning them you’ve copyrighted it and they’d better not steal it. Editors and agents already have a working knowledge of copyright and the legal costs of copyright infringement. They don’t appreciate amateur writers covering every page of a manuscript with copyright information.

Writers also tend to be more paranoid about copyright infringement than they need to be. There are so many ideas out there some will naturally overlap, but direct plagiarism is rare. An idea is only as strong as its execution, and if the writing isn’t professional, no one will go to the trouble to steal it.

If you still feel the need to include the copyright symbol on your submission, simply write © followed by your name and the year the manuscript was written at the top right corner of the first page of your manuscript.
--Jerry Jackson Jr., assistant editor, Writer's Digest