To my amazement, the Myth Which Will Not Die has again reared its head at one of the handful of writing sites I visit often.
Can't I just mail (or email) a copy of my novel/script/poem/essay as proof of when I created it, without going to the bother and expense of copyrighting it?
Sure you can, but in the United States this will not prove anything at all in a court of law. No US court at any level has recognized this so-called Poor Man's Copyright. The website of the Copyright Office at the Library of Congress even says so, yet people who should know better still pass this information around as if it has value.
The moment you save your writing to any fixed medium--your computer's hard drive, a CD or flash drive, a print-out, handwritten sheet of paper, a cocktail napkin--in the US, it is copyrighted. Registration of that copyright offers further protection and is not free, but it is not required for your work to be copyrighted.
For whatever it may be worth, violations of copyright for monetary gain are fairly rare. What's increasingly common is the theft of writing posted online for reposting elsewhere, without permission or acknowledgment. That's a pretty compelling argument against posting in an open public forum, whether you're in the US and that work is automatically copyrighted or not.