People are humans...meaning, they make a lot of mistakes.
Therefore, I suggest that members be allowed to edit their posts after they are posted.
A lot of times people will post something then after they'll want to change it a bit--either to fix a mistake or to add/subtract something from their post. Instead of them having to make another post--they should be able to edit their original post.
Along with editing--members should also be able to delete posts. For instance, if a member accidentally makes a double post-they should be able to delete their second post. Also, if a member makes a post then decides that he doesn't want to post that piece of information then he should be allowed to delete it as well.
The main purpose of this is so that all the members of the forum will be able to verbalize their thoughts in the clearest way possible. People don't like it when "there's no going back"...meaning, once you post something, there's nothing you could do to change it.