In the U.S. (and everywhere else), you automatically own the copyright, simply by creating an original work. However, you cannot sue anyone for financial damages from a financial loss if you don't register the copyright with the U.S. Copyright office. However, rather than $35, it's actually $55, and that's for each work that you produce. So, if you have five books to your name, that's an incurred cost of $275, which is not an insignificant figure.
Now, it's guaranteed that you won't need to sue anyone for each piece of fiction (or non-fiction) that you ever write, so you'll need to weigh the chances that your financial losses will surpass several hundred dollars for every piece you ever write, or is it just not worth it?
Even if you can't sue for damages, you can still file cease and desist letters, to stop a work from being abused. What's more, even if someone steals your work and earns a million dollars from it, you'll have a hard time arguing that your accumulated previous sales of $50 were 'damaged' by the copyright thief's work. In fact, the court might argue, that he actually boosted your sales, rather than hurting you at all.
But, in the end, it's your choice how you waste your money!
Oh, in most cases, you have a window where you can retroactively file for copyright (within so many days after a copyright abuse takes place), so that plays a role in your decision too. Though often, you never learn of it in that limited window anyway. :(