Sean,
You'd need to open up the campaign editor to see if the level of victory is significant in determining what happens next. Sometimes it is and sometimes it isn't. It's all detailed in the campaign file for each step of the campaign. Sometimes a draw is as good as a victory, and sometimes a minor defeat is no worse than a major one. It's all up to the designer and the situation he is simulating.
But yes you are right, if you show mercy to a retreating enemy in one battle you may end up with a more difficult task in the next battle, not necessarily because of the implications of the victory level, but because you'll be facing a bigger enemy army with less fatigue. Not that any of this will worry a general of your stature.[:D]
[url="http://homepage.ntlworld.com/a.r.barlow/acw/acw.htm"]General Antony Barlow[/url]
[url="http://homepage.ntlworld.com/a.r.barlow/aoc/Western_Theater.htm"]Commander, Western Theater, Union Army[/url]