Hi Dave,
This is an interesting one for me!!
Do you mean, sign your work to add your personal, individual touch to the work, or sign as a proof of copyright, to protect against fraud?
I ran two studios, one shot commercial, the other social (weddings, portraits, etc.)
In commercial shoots, we handed over the originals (transparencies)to the printer so copyright was assumed by the commissioner of the work. This was normal and we invoiced accordingly. In the social market copyright remained with the studio. The photographer would sign any prints ordered from the negatives, to add an individual touch. All prints would be stamped with our copyright on the back.
I also receive occasional payment from stock agencies on a pay per use basis. (Copyright remains with me.)
I have, in the past, been subject to copyright fraud in the social market. Proving it however is a different matter, and a very expensive road to go down.( Personnel experience.)
So, I guess signing and stamping your images is nice, but to anybody other than friends and family, its open house. Especially now in the digital world.
On a personel front, if you give your work to people, or they ask for copies, I would sign it with pride.
Andy.