I don't know if there is a canonical policy somewhere hidden away, but I generally ask myself if the effects of CW'ing a question are desirable:

The main effect is of course that no one gains rep for being upvoted. Is that desirable? Rep is meant to reflect a users helpfulness and level of knowledge and general contribution to the site and the community. If someone writes a good answer to the question, does that indicate a valid contribution?

If so, it should be reflected in their rep score.

Conclusion:
A question should not be marked CW if it is possible to write valid, helpful and knowledgeable answers which contribute to SO.

Even if there is more than one valid answer (in open-ended questions), individual answers may still have value, and so they deserve the rep gain when they're upvoted. Marking such a question CW just discourages people from putting any effort into their answers.

The other consequence of CW is that posts can be edited by more people. Usually, I don't think that's a big deal. I think that's a vestigial property, it was useful before the creation of Meta, back when CW posts served as a repository of howto and faq's about SO itself. Today, I can't think of many cases where it's desirable.

I know this brings me at odds with a large group of people who seemingly get a kick out of demanding "THIS SHOULD BE CW" whenever there's even a hint of subjectivity to a question. I still don't understand why, but it seems to be a cult created around some vague statement Jeff may have once made back when the site was in its infancy.

Mark a question CW if answers have no correlation to their authors' contributions to the community. If it is a poll thread, one answer is as good as another. The value is in the entire set of answers, not in what individuals chose to vote. So mark it CW. If it is a joke thread (although those seem to be getting rare), mark it CW. There may be good and bad answers, sure, but none of them say anything about the answerer's technical skills or contributions to the site. No matter how funny my joke was, it doesn't mean I'm any better at answering questions on SO, or that my account should be given broader moderating powers (as happens when you gain rep)

But CW is grossly overused in my opinion. Outside of joke/poll style threads, I really can't see a use for it any longer. It was largely made obsolete with the creation of Meta.

When to do it

The main (perhaps only) reason to mark your own question as CW is if you post a topic which you intend from the begining as a Wiki topic.

Other types of question which should be marked CW from the begining, if they are even asked at all, are:

• Trivial 'poll' questions: for example, "Do you use version control? {yes/no}"
• Questions intended as a 'joke': though such jokes are increasingly rare now on StackOverflow
• 'Soft' questions, whose answers neither reflect nor contribute to people's professional and technical know-how.

However new poll questions often get closed, as do jokes, discussions, and 'soft' questions, whether or not they're marked as CW.

When not to do it

Except as above, questioners need not mark their own questions as CW. Instead, any and all questions that are worth asking and worth trying to answer are worth potentially being given rep for. In particular, the following are not good reasons for marking a question as CW.

Questions which can have more than one answer -- The FAQ says, "this is a place for questions that can be answered", but it doesn't forbid questions which have more than one answer. Questions whose answers are subjective perhaps shouldn't be asked at all (especially if argumentative), but otherwise a question's having more than one answer is not a reason for the questioner to make it CW (except when it's a "trivial poll" question as mentioned above).

Questions which will be downvoted -- some questions will be downvoted for one reason or another. In this case you should ask a better question or not ask it all: don't make the question CW just in order to escape being given negative rep.

Questions which will be upvoted -- some people, when they don't like a question for one reason or another, demand that the questioner should mark the question as CW so that the questioner doesn't get rep for it; however:

• If you don't like a question then, instead of demanding that it be CW, it's better to ignore it without upvoting it, or downvote it, or vote to close it.
• Upvotes for questions are only worth +5 now (no longer +10): so, question's being upvoted isn't so important any more
• There are other mechanisms (e.g. sufficiently many edits, or moderator actions, as listed in the FAQ about CW) which allow a question to become CW later: there is therefore no need to demand that a question be CW from the begining.

Questions which become FAQs -- people used to use CW to create the FAQ topics: this kind of topic and discussion has since been migrated to Meta. Outside of joke/poll style threads, CW was largely made obsolete with the creation of Meta.

Summary

A question should not be marked CW if it is possible to write valid, helpful and knowledgeable answers which contribute to SO. Even if there is more than one valid answer (in open-ended questions), individual answers may still have value, and so they deserve the rep gain when they're upvoted. Marking such a question CW just discourages people from putting any effort into their answers.

• 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 when it's first asked.
• It's virtually never appropriate to tell the questioner to mark their question CW.