Rotating troops at the front of battle

[Ibrahim]

as romani, all the time; usually though, my attacks are so furious, that the enemy routs before I need the second line

no discipline with the enemy..especially with a horns and breast style attack (of a cattle-its a zulu way of attacking)

[Intranetusa]

I usually do not rotate units at all.
I've found that the best strategy is to have all of your units engaged at once - ie. such as surrounding the enemy

Having all of your units engaging the enemy army at different spots is usually more effective than keeping some troops in reserve and rotating troops.

However, rotating units is more effective in fighting defensive battles, battles you're outnumbered in, city defense, or battles consisting of a lot of troops - where micromanagement & having all your units engaged would be difficult.

[hoom]

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.

[Mediolanicus]

I use it all the time, but sometimes it may cause extra injuries.

I've disabled fatigue because the AI always charges the last mile to my army and I grew tired of fighting exhausted armies, which caused terrible morale and easy routings.

Now I have to roleplay my own troop's fatigue (by rotating them for example) and my battles last longer and are more fun!

[Recoil]

I do it quite a lot, mainly when I've got enough space to move my men about.

As for the problem with men dropping likes flies with their backs turned to the enemy, I find that putting on 'guard mode' means that the units at the back leave first, and a small rear guard stays to cover the others. While it still does lose men I prefer using this method rather than just simply having up to half the unit decimated as they all scramble to get out of there.