Many forums use a last in, first out (LIFO) order for displaying successive posts. This puts the most recent comment at the top which, IMHO, makes it easier to use. Those who need to know the antecedents are probably used to "digging down." What would the majority like and is it even possible with Drupal (I used to use in in Word Press).
The AAVSO forums used this method originally, and it was widely loathed. In general, putting most recent posts first works fine if everyone is responding to the OP, but makes extended discussion more awkward. If there's a lot of demand for it, we might reconsider it, but our experience so far has not been good.
Also, when there are posts in a thread you haven't read, it tells you in the forum; you can click where it says "1 new post" (or whatever) and it will automatically take you to the most recent post you haven't read, which makes it easy to follow ongoing discussion.