If you wish to register the image, suggest you collect digital images and make a collective singular file with all images combined. This will save you money as you could have 20 or 30 images in one and by registering the lot as a singular work of multiple images you are registering all images.
Commercial photography has basically 2 work conditions. (1) photographer retains rights (negatives) therefore your client theoretically should return for more prints. Image can not be used by anyone else. (2) "Buyout" client pays extra, maybe double, and receives negatives and all media created. You may have the right to use the imagery to promote your business but that is where it probably ends.
Possession is 9/10's the law. Unfortunately clients do not always see it that way and feel they can do anything they want with it, and usually do. Problem is if you want them to remain a client do you make a stink about it? Typically I like the buyout. I make more money
and usually the negatives will get mishandled and become unusable, or lost. Where do you think they will go.
Any digital image of a print that I put on the web I put a copyright on it. People, especially polliticians love to surf looking for images for campaign flyers, etc. (gotcha) Whether you register now or when the infraction takes place it should not matter. Internet law has a "fair use" clause whereby you can copy the image as long as it is not going to be used for financial benefit, or used to denigrate a person, business, culture, etc. publicly.