• If I just correct a simple spalling misstake in normal text, I don't bother pointing that out (unless it was pointed out to me, and an official thank you is appropriate).
• If I clarify my point, adds whole sentences or change the "spirit" of the post etc, I generally do that in an Update: block, or point it out from a such.
• Changes to code I almost always point out, the exception being if I catch it immideately after hitting submit, I may just correct it fast, trusting the odds that noone had the time to copy or download it yet.

Some people tend to write whole changelogs, some even timestamped for every change they make, including typo edits and other stuff. While I am too lazy to do that myself, I kinda like reading those. It shows that the post is alive and maintained by the owner. Oftenly I guess it is overkill, but if one has the stamina to do such, keep it up. :)

I think pointing out edits is mostly only necessary if it would mean something to anyone reading it both before and after the edit in question. But it is also a nice tool to show how you were thinking from the start, and how that thinking might have changed - it is a part of how you write, in a way.