If you intend to reply to one specific posting in a discussion, please hit the "reply" button beneath that posting. By doing so, you assure that your posting will be tied to the posting you're replying to.
If you post via the "post a reply" box at the bottom of each discussion, it will be hard to know which posting you're replying to. It will look, for onlookers, like you are replying to the first posting in the discussion.
If you intend to reply to that first posting, the "post a reply" box is fine to use.