This decision is entirely up to you. Making a post **community wiki** will result [in the following][1]:

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

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 *your* answer.

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 **continue improving your answer**, then by all means, make it community wiki, and leave a note for others to see. This would encourage contributions.

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


  [1]: http://meta.stackoverflow.com/questions/11740/what-are-community-wiki-posts