This depends on the type of traditional publishing you are seeking.
If you are signing with a "Big 5" publishing house, they will typically be responsible for seeking and securing copyright. It is generally stated in your contract that they will provide the copyright protection and registration.
If you're self-publishing, you're not generally required to copyright your book ahead of time. However, doing so can be beneficial in protecting your intellectual property and ensuring that any potential infringements on it can be legally challenged.
If you're looking to publish with a smaller "indie" press, it's likely that the publisher will ask you to register copyright before the book takes off. This guarantees that you are the exclusive copyright owner and gives you control over who can use your work and for what purpose.
In any case, registering your copyright is important for your intellectual property. You may also want to consider filling out a copyright form with the U.S. Copyright Office https://www.copyright.gov/. This will ensure that your rights are protected both in the U.S. and internationally.
