No, my experience at Cons and author signings are that they are happy to sign any book or even some people have posters, napkins etc.. signed if they don't have any books or forgotten to bring books by a specific author etc. It's not like a standard author signing/appearance where sometimes it's a part of a promo tour and the expectation is more that people are there to have the latest book/series signed etc. In other words it's pretty flexible as per what occurred when I was in Japan for WorldCon '07Oh that's great news - Celia and Fiona at the Convention! I shall have to bring one of their books in the hope of getting it signed - or is that done for only new books?
Strange I thought it was only going to be a Science Fiction Convention
NO WorldCon has always been for both SF and Fantasy but with a greater emphasis on SF but there's still a decent % of Fantasy represented in various activities and author appearances etc.. Whats not strongly represented is the third main leg of Speculative Fiction, namely Horror. The have their own conventions for that. Then of course there's the graphic novels and comics which are represented in a sense by the artists who attend WorldCon incl. an exhibition/gallery of work people are invited to take a look at and usually the prints etc. are for sale. That's often housed in the Dealers room where you also find the SFF booksellers. The comic people have their own major convention in Comicon.
I hope that makes things a little clearer.