I suggest having the Edit-button available permanently. It makes posting so much easier.
One example of when it is to great help is Grahams post, where you want to want to change obvious mistakes. Another is when you make a step-by-step or a How-to something. Then you will get comments and Qs, and want to clarify your long winding instructions, or add pix. This is best done in the original post, so that one doesn't have to read 50 posts to get the full picture, but only the first.
When editing a post, there will be an automatically generated comment at the bottom of the post saying "Last edited by (user) at (date & time)". And then it is the up to the poster to post in comments like "Edit: Added points 6a & 6b" or "Edit: Further to the above, blahblahbla".
Other forums do have the poss to Edit at any time later, and it seems to work wonderfully. Two examples are mud (big forum) and our local Norwegian LC club forum (small forum). The latter is even using the same software, by the look of it.
Links:
Mud thread edited one year after initial post:
http://forum.ih8mud.com/100-series-crui ... t-diy.html
NLCK thread where a user has taken on the task of maintaining links-pages for all kinds of LC related stuff:
http://nlck.no/forum/viewtopic.php?f=25&t=7974