the conclusion of battle is something like this (correct me if I'm wrong and I know there are some rule variations that actually change this drastically):
tally up all hits.
remove casualties
add support where applicable
determine the winner
because the winner is AFTER casualties I think dave is right in using that frontage to determine which direction to fall back. I believe it is part of the strategy a general can use to pre-determine his troops fallback direction. Same goes for a general trying to get the most support out of his troops by strategically removing the NON supporting or NON supported infantry stands.
In death you can still achieve a great many things!

lastly the rulebook talks about a "retreating unit's" direction, a unit doesn't become "retreating" until after support is added up for a final score to determine a victor, and support is after casualties (in most cases), hence my initial comment.
As a dwarf player I have studied these parts of the rules but I am certainly not an expert or have nearly the experience a lot of other players have. HA!