I just (attempted to) reject an edit to include the "self-study" tag in a question, because the editor was "pretty certain that this is a self-study/homework question". "Self-study" is I think the only tag that refers to how the question relates to the OP, and not to the content of the question, and I don't think it is appropriate to add it to a question through an edit - we should ask the OP instead.
I am posting this because while I was writing the reason for rejecting the edit, somebody else accepted it. So I figured, there are diverging views on the matter, and hence room for discussion.
So the question is: "Should we add the "self-study" tag through an edit, or should we always ask the OP to do so?"
Response to Andy W comments and the "duplicate" issue
As is usually the case, Whuber's answers are exemplary, but he too writes "In my experience, around half of the questions the community initially believes to be homework turn out, in fact, not to be so". Since the way we approach a "self-study" question has differences with any other, I believe that a 50% chance of being wrong is enough to validate a policy of "always ask first".