Filing for copyright registration with The Copyright Office gives you more options for challenging someone who uses your work without your permission. While registering your work is optional, you have a more powerful and solid claim for copyright infringement against someone if you have a copyright registration.
Even if you have given someone permission to sample or use your work in their own project, having your original work copyrighted (and your agreement in writing) is important for the same, simple reason that you will have a more solid copyright infringement claim. For example, if the folks you are collaborating with go against their word or try to alter the terms of your agreement after the fact… a solid claim for copyright infringement can be a HUGE benefit. (It’s pretty common to be happily collaborating together today and to fighting over rights and money tomorrow.)
The good news is that copyright registration is simple and inexpensive. For details check out The Copyright Office’s website: http://www.copyright.gov/