If you are the last person to post in a thread, you need to either edit your previous post, or remove your previous post immediately after you create your new post.
If you are writing an "update", this is the same as a "bump" and the same rules apply.
Should this really apply in all cases or it should be considered based on the situation. Signature threads require the managers to post payment updates weekly, and except there was a open spot, they are likely the most recent comment on the thread, merging such replies could get bulky and difficult to manage, as one would not know when any post was made besides the most recent edit.
I would not see it as a big deal if you post in a thread in the altcoin sub that results in its bump score increasing. If you are being critical of the project, anyone viewing the thread will see your criticism.
Bad publicity is still publicity.
Also, most likely, the OP would be more viewed that the most recent comment several pages down.