Personaly I use cavalry to:
1. Guard the flanks of my centre (infantry)
2. Engage other cavalry
3. Chase routers
4. Engage isolated enemy units (e.g. un-guarded artillery, skirmishers...)
5. Guard the rear of my skirmishers or other flanking units like horse arty (also deployed to the flanks of my centre)
6. If i can, and only if the melee is deep enough, I will flank or charge the rear of the enemy centre.

I've noticed that when attacking/moving through infantry, it is possible to avoid friendly casualties by avoiding attack orders (e.g. double clicking on an enemy unit) by double-clicking on the ground instead.