Now, multi-series franchises are more complex. For instance, when I speak about Doctor Who, I mean Doctor Who only - without any of K-9 and Company, Torchwood, The Sarah Jane Adventures, etc. being involved. When I want to include these spin-offs, I say that they are in "The Doctor Who Universe." This allows for me to distinguish between them for typical conversations with the widest variety of fans. Someone who does not know diddly squat about the Prime Computer ads should not have their fandom of the Doctor Who series questioned. Similarly, if Doctor Who had a romance-based spin-off, I would not be interested, but that does not diminish my knowledge and understanding of Doctor Who the television series itself. That's also why I don't include the old annuals, comics, novels, etc.
In iCarly, therefore, we have the show iCarly itself and the universe that includes iCarly, Victorious, and Sam & Cat. So canonicity in Sam & Cat does not direct the series continuity of iCarly, but it does affect the universe continuity of iCarly. I consider material from spin-offs to be "essentially canonical" by the examples above. For full canonicity, it must include any eponymous characters (so, Carly), must explicitly be referred to as iCarly with the official media materials, and contain more than 50% of the main characters (so Carly plus at least two of Sam, Freddie, Gibby, and Spencer). Similarly, for Victorious, I require Tori plus at least three of Trina, Jade, Cat, Andre, Beck, and Robbie for potential full canonicity.
Note what this implies: actions within any of iCarly, Victorious, and Sam & Cat affect the canon of the universe, but need not directly affect the canonicity of the other series within the universe. This is because someone need not be a fan of the other series to discuss a specific one. I figure more passionate fans should be able to keep to stronger restrictions than less interested ones.
Which leads us to the most contentious part of the canonicity debates: what do you do with flat out contradictions?
Sometimes there is nothing you can do: in "The Nanny," Fran Fine could have been born any time from 1957 (Fran Drescher's real birth year) to 1963. It is so contradictory that the best you can do is throw up your hands and give up.
Sometimes, you can make a "retroactive continuity" fix to the canon, better known as "retcon", which can be either a verb or a noun. For instance, I can retcon Freddie asking Sam out and then celebrating the Creddie kiss by saying, that he is a hormonal teenage boy with "better than average looking" female friends who has divided feelings. This does not excuse what he did; it merely makes his behaviour consistent with established facts via a consistent assumption.
Another retcon is to fix the contradictions between the supposed birth ages of the characters given on this site, their actresses' and actors' actual ages, and their purported ages in the show by saying that the characters aged less in the shorter seasons.
As you can see - the more encompassing and more general your retcon, the more strained and less believable it becomes. That's why my canonicity debating rules includes one last set - when you have a choice of retcons: choose one that does not pick a side in a fan fight; chose one that requires as little retconning as possible; choose the one that contradicts as little continuity as possible; in any multi-series franchise, choose retcons that affect as little of the franchise as possible.
Now, you may not agree with this, but it gives me a framework to debate canonicity amicably with a large number of fans while getting input from the largest number of fans that I can. Whatever you choose, make sure that it works for you.