So long as it involves the actors, I'm open to it being canon. Say if there was a Buffy movie

But Joss writing a comic or novel, I'm less sure about. It doesn't seem to have much to do with the actual show, not when the Buffy tie-in stuff was always treated as extras, and often contradicted the main storyline on the show. The original description of Spike killing two slayers for instance. I never got the impression that novels and comics were supposed to be Buffy canon, or that Joss ever even read them