Whether on Reddit or a pre-Reddit forum, I usually never bothered to add an 'ETA:' footer for small spelling or grammar changes, or for larger changes edited in immediately after posting (as an author I don't exactly have the most ordered thought process...). Anytime I added information more than a few minutes after posting -- for example, providing citation links or adding nuance to something that didn't come across as clearly as I intended -- I would usually provide a footer summarizing those changes for posterity. Unfortunately, reddit being reddit the discussion would usually have moved far past my post at that point.