I saw your statement not to copyright your material. The 1976 Copyright Act applies to original works whether published or unpublished, as soon as created and put in any reproducible form. The work does not need to be registered with the USPTO to be protected under state and federal copyright laws. Formal registration with the USPTO is beneficial in that treble damages and attorney fees for infringement, inter alia, are available.
For starters, let's be clear about who you register with.
It's the US Copyright Office, part of the Library of Congress.
USPTO is the US Patent and Trademark office.
Patents and trademarks are NOT the same thing as copyright.
USPTO is a federal agency.
And, moreover, copyrights can be assigned to a publisher, so I am very curious about your statement.
Secondly, you never EVER assign your copyright to anyone, particularly not a publisher.
Not even if they ask nicely.
Copyright is ownership.
A publishing contract is a license to publish. It does NOT transfer ownership.
A publisher "buys" your book only in shorthand. What they actually do is license the right to print it.
Here's a screen shot of a publishing contract:
(This is one of those terms that a savvy agent looks for in a publishing contract, particularly those from small presses or periodicals.)
If you see a publishing contract that requires you to transfer your copyright, do not sign it.
You're essentially surrendering complete control of your work.
Publishers that talk about "Big 5" publishers taking your copyright are either ignorant of how publishing works, or trying to use your ignorance to line their pockets.
But the real question you want answered here is why I advise writers not to register their copyright before the book is published.
Simple: once you register, that's it.
If a publisher comes along later, the Copyright Office lets them know the book is already registered, and oh by the way, this is a copyright amendment and they charge you more than they do for the initial registration, and it's a big old pain in the asterisk.
If you self-publish your work, you DO need to register the copyright.
But if you're querying, you really don't.
If you have, for whatever reason, make sure you tell your agent when an offer is received from a publisher.
Like a lot of things, it can be handled easily IF the agent knows about it.
Any other questions?