Unlike almost any other element of copyright law, there's actually some straightforward information about how copyright registration works that could, in fact, enable you to make a pretty clear decision about your own course of action. Wild, I know!
Registration is how you "copyright" something, right?
NOPE. The use of the word "copyright" as a verb is outdated. Registration is unrelated to ownership of a copyright.
A human being capable of expressing herself in almost any way already owns copyrights. Copyright attaches automatically the minute a work is "fixed in any tangible medium of expression", under U.S. law. Most other countries have similar laws in which a copyright comes into existence at the same time the copyrightable work does. (Whether a work is copyrightable (i.e., the kind of thing copyright applies to), and whether it is "fixed in a tangible medium" (i.e., recorded somewhere, even if fleetingly) are technical questions I'm not gonna get into here. Both are addressed in § 102 of the copyright code, for starters.)
Then why do people bother registering? What does it do?
Registration provides some important benefits:
First and foremost, registration is a prerequisite for any civil infringement suit - no one can sue someone else about a copyright without first registering that copyright. However, registration can take place at any time during the existence of the copyright - and can even take place after a copyright holder finds out about an infringing use.
Second, registering before any infringement takes place makes sure that certain legal remedies are available. Specifically, registering within three months after publication and before any infringement takes place means that statutory damages and attorney's fees are on the table in a lawsuit. If registration happens later, those remedies are not available.
Attorney's fees are pretty straightforward - the winner of a lawsuit can ask that the loser has to pay their legal fees. Statutory damages are a little more complicated - basically, the plaintiff doesn't have to prove they lost any money (which is what damages usually mean) - they just have to prove that infringement took place, and they get a pre-set amount of money ($750 - $150,000 per infringing copy).
Finally, registering your copyright soon after publication creates a presumption both that the copyright is valid, and that the registered owner is the real owner. This isn't usually a big issue, but the effect of a presumption in court is that anyone who wanted to argue that the copyright in fact belonged to them would have to prove that the registered owner was not the real owner.
That all sounds serious! I should register!
Well, registration does cost money ($35-$160 US.) And it isn't required. And the creator absolutely owns the copyright automatically. And registration is almost only ever relevant to lawsuits about the copyrights. So some people do choose not to register - and many more simply don't realize they can.
Even creators who don't ever register still own their copyrights, and can distribute, license, or sell their work secure in the knowledge that they are legitimate copyright owners. As owners, if they discover doing something with their work that they believe is infringing, they can still send completely legitimate letters requesting that such uses be stopped. And if that doesn't work, they can register after the fact.
Even owners who register after an infringement has occurred can still bring a lawsuit. In a lawsuit, they can still get "injunctive relief" and "actual damages". That is, the court can order an infringer to stop what they are doing, or even to destroy all existing infringing copies. And the court can require the infringer to repay any actual monetary or business losses. These can include lost opportunities, but unlike with statutory damages, the copyright holder has to prove that they actually did have those losses.
Someone else says they'll register my copyright for me...
A third party registering copyrights on behalf of creators might simply be doing a nice thing - providing a service. But if they're charging for it, it might be something the creator can do herself for less money (it's currently $35 to register your own copyright online.)
A third party offering to register copyright on behalf of a creator might also actually mean that they're going to register it for themselves - if a creator transfers copyrights to someone else (such as a publisher) then that someone else will be the legit owner, and that someone else will be the one getting the benefits of the registration. If the creator wants to transfer her copyrights to the third party, fine; but if she's transferring her copyright because she's been told she'll get some benefit from the registration as a result, that's misleading.
If I register, do I have to give a copy to the Library of Congress?
Technically everyone who publishes anything in the U.S., registered or not, has to deposit two copies with the Library of Congress within three months after publication. Failure to deposit does not affect the existence of a copyright, but it does actually expose the copyright owner to the possibility of a fine! There are some exceptions for smaller print run publications and such.
So should I register my copyright, or not?
Every creator has different goals and intentions for their creations. There are few downsides to registering. Considering the benefits and costs of registration, and how those benefits and costs align with the creators' goals, can help with the decisionmaking process. So can consulting your own lawyer.
Please do note that there are a couple more complicated wrinkles around registration (different benefits) for works protected under the Visual Artists Rights Act (primarily works of visual art that could be described as "fine art") and broadcast transmissions.
There's more info about the options available for registration (although it's a bit out of date on the online options) and about how an enforcement lawsuit works, from the Stanford Copyright and Fair Use site.