Just make sure that you use Alt button for retreating units so they dont wheel around before marching off.
Oh my god, all these years without knowing you can do that

For the above reason I don't often rotate in actual combat.

But I do often switch troops from parts of a line that have been depleted/tired out with fresh unengaged troops either from a different part of the line or reserves.
Also, in city assaults its often very important to use fresh troops to push forward after the initial bridgehead & for the city square if there are some tough troops there.

A powerful but exhausted unit can lose a lot of good troops when engaging a fresh enemy.
Its often better to use a fresh but weaker unit.