<h1>When to do it</h1>
<p>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.</p>
<p>Other types of question which should be marked CW from the begining, if they are even asked at all, are:</p>
<ul>
<li>Trivial 'poll' questions: for example, "Do you use version control? {yes/no}"</li>
<li>Questions intended as a 'joke': though such jokes are increasingly rare now on StackOverflow</li>
<li>'Soft' questions, whose answers neither reflect nor contribute to people's professional and technical know-how.</li>
</ul>
<p>However new poll questions often get closed, as do jokes, discussions, and 'soft' questions, whether or not they're marked as CW.</p>
<h1>When not to do it</h1>
<p>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.</p>
<p><strong>Questions which can have more than one answer</strong> -- <a href="http://stackoverflow.com/faq">The FAQ</a> 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 <em>subjective</em> perhaps shouldn't be asked at all (especially if argumentative), but otherwise a question's having <em>more than one answer</em> is <em>not</em> a reason for the questioner to make it CW (except when it's a "trivial poll" question as mentioned above).</p>
<p><strong>Questions which will be downvoted</strong> -- 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.</p>
<p><strong>Questions which will be upvoted</strong> -- <a href="http://meta.stackexchange.com/questions/19478/the-many-memes-of-meta/19536#19536">some people</a>, 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:</p>
<ul>
<li>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.</li>
<li>Upvotes for questions are only worth +5 now (no longer +10): so, question's being upvoted isn't so important any more</li>
<li>There are other mechanisms (e.g. sufficiently many edits, or moderator actions, as listed in the <a href="http://meta.stackexchange.com/questions/11740">FAQ about CW</a>) which allow a question to become CW later: there is therefore no need to demand that a question be CW from the begining.</li>
</ul>
<p><strong>Questions which become FAQs</strong> -- people used to use CW to create <a href="http://meta.stackexchange.com/questions/tagged?tagnames=faq&sort=votes">the FAQ topics</a>: 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 <a href="http://meta.stackoverflow.com">Meta</a>.</p>
<h1>Summary</h1>
<p>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.</p>
<ul>
<li>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.</li>
<li>There's usually no reason for a question to be CW when it's first asked.</li>
<li>It's virtually never appropriate to tell the questioner to mark their question CW.</li>
</ul>