I have two AA prints which are signed on the matts and on the back of the print. I have a Paul Caponigro print that was signed on the overmat and no where else, and two Edward Weston prints that aren't signed at all.
Oh, yeah and then there's JP Witkin - who's totally variable depending upon the edition. My advice -
If the print is dry mounted (eschewed today by the archivists), sign the the mounting board, below the photo on the right hand side. Also, sign the mounting board on the back. The overmatt can be cut larger to show your signature.
If the print is held in place by photo corners or archival tape, sign the print lightly on the back with a pencil (HB). not hard enought to deform the front side of the print. Then, sign the matt below the print on the right hand side. If the print is seperated from the matt the signature is still on the back of the print.
Wow! - I just wish I had this problem.