Since 2004, all my images have been registered. It took years to scan those on film, and when I found out how easy it was to do a group registration, all my future registrations were a piece of cake.
Suggestion-- Why not just register everything as published and be done with it. That way you are safeguarded with registration.
Remember, the Copyright Office's definition of "Published" is different than what we use in the real world. If you get paid for shooting photographs, then, by definition, it IS "Published."
Listen to Carolyn and others. If you don't register, you can and WILL loose out. I may sound like a commercial, but trust me, it is better off being safe than sorry. If you don't register because it is time consuming or you feel it is NOT worth the effort, then I will NOT feel sorry for anyone. Be professional! Treat photography like any other business.
Even if you are not a professional, copyright your photos and videos!!!
As the Nike commercial says, "Just DO IT!"
The stumbling block for a person that has not registered on a regular basis and who would like to start doing so is the nightmare of trying to figure whats been published and what has not from past images. If you could just collect all your photos and register them that would be simple. Having to separate them is not.