Not really. The Del Rey novels and the DC comics are canonical, but the first line of novels from Dell is not, except for The Shadow Within and To Dream in the City of Sorrows. The Dell novels were meant to be canonical, but JMS found he couldn't give them as much direct supervision as he wished while the show still demanded most of his attention, so almost all of them ended up having continuity problems or differences in their interpretation of the universe. I recall JMS saying that all of them have some "canon value" to a greater or lesser degree, but most of them are considered effectively apocryphal, and the two I mentioned are the only ones that were reprinted by Del Rey and alluded to in later books. (Note that City of Sorrows was written by JMS's wife, so that was the one case where he and the author were able to communicate routinely enough to keep it consistent.) The Del Rey novels were published after the show ended, so JMS was able to oversee and outline them personally, to be a proper "showrunner" for them, and thus they count as canon. He also outlined or wrote most of the comics. I think the comics managed to be canonical despite being put out while the show was on the air because they take less time to write and publish than novels, so he was able to devote the necessary attention to them.
That's all that "canon" really means -- it's a fancy word for the creator's own work, their own version of their creation, as opposed to other creators' works derived from it. Or, in the case of a long-running franchise that passes through multiple creators' control, it means the work created under the auspices of the copyright owners rather than their licensees. As a rule, tie-ins only work as an extension of the canon if they're directly developed and supervised by the same people who create the canon. Because any two different creators dealing with the same universe are going to see it and approach it in different ways, so the only person who can really create something that fits perfectly with the creator's own vision (and thus has a reasonable chance to avoid later contradiction) is the selfsame creator. So the canonical B5 tie-ins are the ones JMS wrote or outlined, the canonical Buffy/Angel tie-ins are the ones Joss Whedon wrote or outlined, etc. It's just a somewhat pretentious label for the creator's own stuff.
