I am messing around on AO3 (instead of doing all the other things I need to do, yes, shush) sort-of catching up on Yuletide fics and sort of thinking about maybe starting to do some actual tag wrangling now that yuletide is over (though I think I need to poke somebody about getting an account for the chatroom) and you know what I have discovered that I have a very strong opinion about, regarding AO3 canonical tags?
If you're working in a canon where somebody's full truename is powerful, and hidden, and binding, and they are very, very careful about who they offer it to and when they use it, even in the privacy of thought --
Then you shouldn't make their full truename their AO3 canonical tag.
It's just wrong. First off it breaks pretty much every rule of magical etiquette, plus it's a breach of trust, plus it's a massive act of hubris. I think *especially* because you're applying it to fanfic - sealing something with a person's truename is to try to make it them; to use a truename irresponsibly can change who the person is, because you're saying that whatever you've given that name to is the person, in some of the oldest Deep Magic there is.
And so sealing a fanfic with a truename - it's saying that your fanfic is the person true, and it runs into the danger of making that true, of shifting the person into the version you've sealed into the fic. And I don't like it. Fanfic shouldn't be about making it true.
...and, okay, yeah, the above is only about half-serious. But if I had fic posted to the AO3 in a fandom where this was relevant, and saw my character name tags turned into truenames? I would not like it. (Luckily the only fandom I currently have up where this applies is Earthsea, and Earthsea has a tradition that a person's truename becomes public-domain regarding stories told about them after their death, and is framed such that most Earthsea stories are in that context, so it's a slightly different situation.)