Personally, this has always been the given for me. As soon as I had completed any script, TV or feature, my first action was registering it. I actually made it into a ritual. Whenever I finished a script, I’d celebrate with a meal out with friends or family. Then the very next morning, I’d drive to the Writer’s Guild with a printed out copy of my script (un-bradded) in a crisp, clean manilla envelope in one hand and pen and checkbook in the other. Just sitting in the room waiting to register with it sitting on my lap while looking around at the movie posters was so inspiring. And the feeling you get when you see them put your script in their fancy envelopes, place it away and hand you that fancy registration certificate… fullfilling. Like when the farmer looked down at Babe and said, “That’ll do pig, that’ll do.” And then I put the exclamation point on by having a ceremonial caramel covered marshmallow from Littlejohns in the Farmer’s Market across the street. And since that’s the only time I have that treat, I’ve kept a trim figure. Well, maybe not trim, but at least I know I’m not fat from Caramel covered Marshmallows.
Point there is that registration should not be a possibility for your script, it should be a given. And even with registration, there’s still always a possibility of someone lifting your work. But you just need to know that you’ve done everything you could to protect your work before you distribute your work.