<p>This decision is entirely up to you. Making a post <strong>community wiki</strong> will result <a href="http://meta.stackexchange.com/questions/11740/what-are-community-wiki-posts">in the following</a>:</p>

<blockquote>
  <ul>
  <li>A much lower reputation (100 instead of 2000) is needed to edit a community wiki post.</li>
  <li>Voting on a community wiki post (up or down) does not affect any user's reputation.</li>
  </ul>
</blockquote>

<p>This might make it easier for others to edit your answer, because it's now community-owned, and they don't feel they're changing <em>your</em> answer.</p>

<p>There are no "criteria" for you to decide whether you need to make something community wiki.  You never have to. However, if you specifically want others to <strong>continue improving your answer</strong>, then by all means, make it community wiki, and leave a note for others to see. This would encourage contributions.</p>

<p><sub>Note: I've made this CW myself, so we have a practical example :)</sub></p>

<p><sub>Note: And I've edited it, so there's that, I guess?</sub></p>