Huh, I didn't know that this was the subject of debate. These are the tiers of canon as I see them at the Foundation.
1. SCP articles and affiliated documents - These are the site's original purpose. As the most presentable documents on the site, they are the closest thing we have to a canon. SCP articles unfailingly maintain a professional tone and a facade of realism. If an entry in the SCP List disturbs the tone of the site or is otherwise unacceptable, it is immediately corrected or deleted outright.
2. Site Info - Help documents on the site for newbies. I put these below the List for the reason that their accuracy is disputed with regard to a few subjects. The general usage of concepts in the List should trump Site Info, which should be corrected to match.
3. Foundation Tales - There are a lot of stories here that aren't exactly plausible, and a few that are utterly absurd. There are many that don't quite reflect the tone of the site, to be sure. Some people may find some Tales more palatable than others. I personally think that any Tale can be dismissed as non-canon. Stories may shed light on some aspects of the Foundation, or perhaps some characters, but I don't think it's necessary that each one have actually occurred in Foundation history.
4. Fieldwork. Other RPs. - Completely non-canon, but inherits canon from the List and the Tales as necessary. I don't think we can trust the integrity of this writing project to spur-of-the-moment dialogue and decisions.
5. Decommissioned SCPs. - As far as the List is concerned, these things never happened. Duke never existed, and neither did 808's boyfriend. There should be absolutely no mention of decommissioned SCPs in current SCP articles. Although it's a given that SCPs in-universe have gone through decommissioning and the numbers freed up for reuse, decommed articles are decommed because they are bad and no trace of them should exist except in Foundation Tales, which are non-canon to the extent that they feature SCP-Ds.
6. Deleted SCPs - Well, it's not like anyone remembers these things, anyways.
Addendum: Concepts may be thought up and developed in role-play, and then, if the idea is good enough and capable of standing on its own, graduate to the wiki. Foundation Tales may be considered on the same level as the List if they follow the same standards, but to make something official, it should be mentioned in an SCP or as part of an attached report.
Addendum: Fishmonger's integration of the SCP-808 entry with Wanderlust stands as an exception, as do other circumstances where the list presently interacts with the Tales.
Addendum: Just because something is canon, doesn't exactly mean it happened. snorlison made the point that many of the more dramatic entries could be lies engineered to impress/terrify scientists working on more mundane SCPs, and documents can be forged. Generally, though, I'd think the vast majority of stuff on the wiki is true: otherwise, things just feel a bit meaningless, if you know what I mean.