Bear in mind, proving you did it first becomes a legal issue, and the best way to do it it to register it properly.
However, if you send it to a big publishing house, and you throw copyright info on there, it's a tad insulting to their ethics, suggesting that they might steal your manuscript.
You don't want any reason whatsoever to have them toss you out before they read enough of it to make a "proper" decision. They WANT to find good manuscripts in there, but there's so many in the pile that it doesn't take much to make them move over to the next one.