With one exception, my prints are signed on the back, well within the edges of the sheet. If possible, I sign with a graphite pencil, which should ensure that the signature and whatever information I put there last as long as the print.
Caveat: I'm not an accomplished photographer with lots of exhibitions and sales, merely an amateur whose prints are in the hands of friends and family. So my choice is reflected by the interest future generations within the family and friend spheres might have in the prints.