I used to be really paranoid about this kind of thing, but as others have stated, the most important thing is that you can prove authorship. It's not usually worth the time, money, and stress involved in copyrighting every single thing, especially for webcomics. Clint Wolf makes an excellent point in that the copyright office doesn't know how to handle these things.
It's probably more important that, if (IF) your comic is ever a huge success, you trademark the name. But even then, for indie and webcomics, I don't feel it's worth it. And taking a perpetrator to court is another huge hassle in and of itself that could cost you a lot more than its worth, even if you win. The truth is, if you're actually a good writer, and if someone steals your idea and gets away with it, you should have two dozen even better ideas already floating around in your head to replace that stolen one. It's harsh, and totally unfair if someone steals your idea, but hey, screw em! Do something even better! Hopefully it never comes to that, though. I've had work more or less stolen, in a manner of speaking, and it sucked and I was very angry about it, but now I just look back at it as a learning experience and since then I have come up with an uncountable list of even better ideas and concepts (wow, I hope that doesn't sound conceited...).
Anyways, life is too short to be worrying about someone stealing your idea.