It's always best to register things when they become defined.
That will not preclude you from copyrighting further developments or variations.
Keep in mind, however, that these days copyright is inherent in the creation af any work of art
Be sure to label any copies (in any form) with (c) & ID of creator(s)/copyright holder(s) &, if possible, contact info.
That doesn't alleviate the need to eventually register for full legal protection options but it does give you substantiation if items come into someone else's possession. They can't claim they had no way of knowing to whom the work belonged.
Keep in mind that except for mass piracy, the government doesn't enforce any of these laws; that's handled in civil litigation (copyright holder sues infringer).
Also be aware that while you can copyright items both in printed or recorded form there are further considerations: recordings are actually a different item.
By that I mean that if you compose something, that can be copyrighted.
If you record it for sale, that can be copyrighted.
If someone else records it, they
can copyright their recording. They still owe the copyright holder of the composition a royalty but they collect royalties on their recording.
Same if you record someone else's work.
You can even copyright significantly different versions (arrangements) of works by others, whether in the public domain or a work owned by someone.
If someone copies your arrangement, & you've copyrighted it, you may be a winner.
Lastly, copyright laws vary, sometimes significantly, from country to country.
At some point it's worth looking into those differences to see how to protect works that may be marketed internationally...especially as we move further into digital/online sales, etc.