I suggest that the questioner should mark their question as CW in the following circumstances:
- When asking a trivial "poll" question (e.g. "Do you use version control? {yes/no}")
- When posting a topic that's intended from the begining as a Wiki topic (like this one)
Except in the above two circumstances, a questioner need not mark their own question as CW (instead, if the question is worth trying to answer then it's worth potentially being given rep for).
The FAQ says, "this is a place for questions that can be answered": but, it says nothing against questions which have more than one answer. A question which has more than one answer (or that's subjective) perhaps shouldn't be asked; but if it is asked, the fact of its having more than one answer is not a reason for the questioner to make it CW.
Summary:
- It's usually sufficient to have the other existing mechanisms which automatically convert a question or an answer to CW, after it's asked or answered.
- There's usually no reason for a question to be CW ab initio.
- It's virtually never appropriate to tell the questioner to mark their question CW.