Ben:
I don't disagree with you that many of us do use our experience with the common "portrait" focal lengths for 35mm film when we consider this issue. The problem that occurs, however, is that trying to covert that experience for use with other formats can be both complex and subject to error.
I shoot several medium format formats, and in at least one case I have to add close-up accessories to my lenses to attain a close up headshot using an appropriate lens. In the end, for me it is simpler to arrive at a single, standard working distance for each intended result (full body portrait, 3/4 body portrait, upper torso and head portrait, shoulders and head portrait, close up headshot) and then choose the appropriate lens to match.