I prefer to have the ability to edit/delete posts, as that way I can ‘control’ what information I have shared on the internet. A time stamp for ‘last modified’ is helpful to help other readers in case of hack’n slash shenanigans that can make threads hard to follow.
Often I like to revisit posts to clarify and/or update the posts (especially if the thread is following an event or activity that is dynamically changing).
However, if for administrative reasons you would prefer to disallow editing/deletion, the expiry/lock-in period should be communicated clearly (in the form of a polite reminder) to the forum user each time they create a post (i.e. above the draft box where it currently says ‘You can use BBCodes to format your content.’, so they are aware of the conditions under which they are posting.
What I would like to see more of in forums in general is the archiving of ‘dead’ posts, so they can’t be necroed. I don’t want them hidden from searching or anything, but rather they should be locked down, perhaps after six months of no bumping/updates and a comment added to the thread along the lines of ‘This post has been archived. you can no longer edit or bump this post’.