Read ScriptMag article on PrimeTime: The Truth about Protecting Your Work by Chad Gervich. This screenwriter/writer is solid. He writes in a tone that keep you glued to the next point he wants you to remember.
Chad mentions that screenwriters who attach a serial number and/or copyright on their scripts are amateurs. It makes the screenwriter appear as a rookie. He says never do it!
Good writers are confident. They don't worry if ideas are stolen or taken. However, protecting a script in its executed form is wise as a timeline to show you own this screenplay, this story.
Definitely check out this article to answer question on script protection. It is a great read!