You must ask yourself (in any instance of creating or establishing evidence that might have to be able to stand up to scrutiny in litigation) will a judge in his chambers or in the court itself, be able to establish the veracity of the evidence presented quickly and easily?
The other aspect of the reality of copyright, is the appearance of evidence. Just how 'imposing' is the evidence? The more concrete the evidence appears, the less likely it is that someone will disregard it and injure your copyright. A book, CD, DVD, whatever that has been published and is available on Amazon, eBay and elsewhere is a clear and very concrete object and the presence of these objects and the dates of publication are very very clearly defined.
Like all property, including your garden or your car, there should be no doubt to any person just what is and is not yours.