I think this all boils down to, if you are a "pure" poster collector, you don't want anything on that poster. To people who just love film, like myself, who would pay that bit more if I knew the signature was genuine. But, For that I personally would want many photo's of the signing taking place, and one where I could see the poster it's self...
I have only ever bought one poster I never personally witnessed getting signed, but the guy I bough it off showed me the photo's of him with the Director, and the Director signing the poster. All the signatures I have obtained on mine Yvonne Has Video taped the event. More for personal reasons than Authentication.
I quite agree with most of what Mark has said, though who you would call C-list in a "B" picture is up for debate... If I like the Actor/Director/Guy who swept up after filming, enough I would get them to sign it. As I have No intentions of selling, these are for my own pleasure, I know they are genuine, and aren't in this game to make money. So couldn't give two F**ks if people don't want to believe if it's a genuine signature or not.
It's the same as anything, if you know your subject you will be fine. If you don't you'll waste money on a fakes......I wonder how many of you "poster" collectors have a Repro in your collection, you bought without knowing it was.......Come on hands up....