Well, considering I have about six different ones, I'd say there's a good chance they're legit. Not to say I'm going to be selling mine...can't get them unless you pry them from my cold dead fingers and disarm the deadman switch in the process.
I got JMS this weekend, on my well worn copy of B5 Magazine, and got the rest over the past few years from Agamemcon and CreationCon...years ago.
During it's prime running, the cast of B5 was making the con circuit a lot, it was real easy to get an autograph on either a photo or cd or whatever. But there are ways of counterfeiting anything these days, so there's always a caution. If they're a legit fan, then you can safely be assured that the autograph is legit.
Personally, it's the people that get signatures strictly for sale that really burn me. Those don't really care that they met the person, merely what they can get for a small scribble on a photo.
::sigh:: never mind...it's late and my personal peeves are peeking through.