There's always so much misunderstanding and fuss over "canon." Let's see if you agree with my basic definitions of canon and its related issues here.
In school I was taught that "canon" was a list of supposedly "good and important books" that one had to read in order to be an educated person. *rolls eyes* In fandon, "canon" has a more specific meaning--Canon is that which the author/creator of a work has established as his intention and the basic "truth." (e.g. , Ichigo has orange hair, Karakura is the town he lives in, Isshin was once married to a woman named Masaki and she died).
Now there are the deconstructionists, whether they know that's what they are or not--the fans or fanfic writers who don't think the author has all authority over the text. Fanfic writers, by very definition, are postmodern deconstructionists in that they take an established story and invent around it or re-invent it. There is also the argument that whatever the author intended, it's all out of his hands now--the story does not have an ultimate "truth" and that there are valid interpretations far from what is accepted canon (e.g., whether or not Kubo specifically stated it, there are gay men in his manga or the idea that the Quincy represent Judeo-Christian culture with an emphasis on the murdered Jews of Eastern Europe).
Then there are the fans/fanfic writers ( waves hand! me! me!) who do not identify with abandoning the concept of authorial intent. If that's your boat, by all means float it, but I think that tradition of the form, interviews with the author, course of the narrative, etc--all factor into making interpretations, some of which have more value than others, and predictions, some of which have more literary likelihood than others. (e.g., Interpreting Tousen as a tragic figure could be considered to have more canon support over his being a true villain or IchiOri is much more likely to happen in the manga than AizenOri). An old-fashioned literary POV but I think it's a little meatier when informed by some of the other weird theories of the past century about what art really is.
And then.... (I'm also in this camp) there are fans/fanfic writers who can abandon canon altogether (oh it is so hard for me--but experiments have proved really fun) and take characters with some basic established traits and dump them in unlikely situations. This concept goes beyond the concept of A/U. It's not just putting characters in another world but it's assigning characteristics to them (deliberately) that a canon purist would identify as painfully OOC (e..g. Urahara seduces Ishida, a minor--I actually read a wonderful story about this!)
A friend once told me that one can write just about anything about any character and make in IC. I'm not sure I buy this, but she said that you have to think of the circumstances in which someone would act OOC. I've done this with a few of my characters (Ryuuken crying in "After You Died and Before That") and Ishida (getting it on with Shinji--and being the seme Ishida! O.O). I don't know if that formula is possible with all characters without putting them in a completely different world and giving them different characteristics. An A/U writer always has the luxury of assuming that whatever world his characters are in, the different circumstances made those characters slightly different people.
Anyway, I hear so much of "that's not canon!" voiced like "you have koodies!" and I think it's pretty limiting to a fanfic writer to stay only in canon timeline (I know--I'm very fond of the characters I love in the world I love--but then you have to write a lot of filler or "in the future" fics).
Writing is one thing--interpreting the manga with "this is canon" or "this obviously is" is very tricky. I'm one of those who believe that pictures in the manga medium carry as much weight as text and that sometimes characters are speaking text that is a lie. Since not everyone sees every picture the same way or gives the same psychological interpretations to characters, it's fairly difficult to prove something is canon when it's not an explicitly stated fact. Some arguments though, I believe, carry more weight than others. I once read where someone said, in all seriousness, that Aizen kissed Orihime in one panel--that it was obvious from the way the characters were standing or something like that. Okaaaaaay. Arguing that Ichigo really likes Keigo although he dismisses him often and treats him badly would be easier.
I'm just putting off writing what I think may be a real-to-goodness A/U and wondering about this sort of stuff.