The registration at a copyright office constitutes a "relative presumption" of paternity, and can always be proved false.
That is, if somebody else registers your work at the copyright office, and you can demonstrate you are the creator of the work (by showing the negative for instance, and having a tribunal technician certificate that it is not a digital negative, while the one of your opponent is ) you can always claim your copyright regardless of a registration by somebody else, or the lack of a registration.
Copyright registration in my country, at SIAE, is awfully costly, slow and unreliable. You have to make some copy of it (print, CD) which goes in a sealed bag at SIAE offices where it remains for 10 years or so, when you have to "renew" it. When you go to court, the CD is unreadable. It's a system thought out basically for books and written material such as music.
The procedure at the Library of Congress, last time I checked, is a bit clumsy.
For "unpublished" work you have to pay a certain tax (with a check IIRC) for each "batch" you hand in. Each "batch" can contained an unlimited, or very large, amount of work but you have to register before "publication" that is, also before giving the work to a stock agency.
That would cause, in the case of a stock photographer, either a delay in giving your work to an agency (if you make two registration per year, you have to wait six months before letting the work out of the door) or to pay any time for a new registration tax. I think I make around 4 submissions per months to agencies, that would be around 50 submissions per year to the LoC, which is a cost.
They have simplified with an online procedure now, but IIRC the basic limitations of having to proceed in "batches" remain.
If anybody can give me better information or advice on how to cheaply register my pictures with the LoC, that's very welcome. The same information is welcome about registration in any other country, such as UK or France or Germany (courts accept any kind of copyright registration by any country having signed the copyright convention, one would normally use the registration to settle out of court in any case).