I had a quick look at one of the first pages on a relevant Google search, and although the steganographic methods are good for proving that the work is yours, when used digitally, for example on a website, they will not prevent the image from being printed. If you can see the image, you can grab the image, and once grabbed it can be printed. It simply won't work as a method of preventing a client from taking the image and printing it themselves. It isn't even going to work if they take the image to have it printed at a lab, since the lab are not likely to go looking for hidden watermarks. In that case you can only hope that the lab considers the images to be too professional looking and refuse the job.
Really the only way to stop people printing the proofs themselves is by making the image very small, I would say at most 600 pixels on the long edge. I know that 600 pixels seems very small, but consider that most people now consider that images displayed at 100 PPI are super duper quality, and so are more than likely to be happy with prints at that resolution too. Oh and to catch the uninitiated I would also set the PPI/DPI tag in the file to something quite high, maybe even 600 PPI, since then many programs are likely to show a very small physical size based on the low pixel count. Even then I would add a large obvious watermark.
If I were selling the client a large print, providing a decent return, then I would maybe send two or three larger sized images, without any big obtrusive watermark, to let them chose the final image. I would only do this once they have paid for the print though, before that it would be the small heavily marked image only.
My personal view is that if you want to make money with pints, you need to make coming in to the studio to go over the proofs etc a part of the process that the client wants to take part in. Once you have sold them the prints then I would give them decent quality digital files too, since then you have made your money.
Alan