Depending on the Leader and if he has any troops.

If I have a leader who leads troops I keep him at the back of the fighting, as if he is injured or comes up against a morale check he can't make he retreats and his whole army does as well.

If I have leaders who don't lead troops more often than not they are used as ferriers and not combat troops, but if they are I put them on a flank if they're equipped, otherwise it's like throwing money away for no noticable benefit.

Giants and other 'good' units are usually above this; as I will use them for combatants, but if I'm going to go through the point of using one to attack i usually equip them, if they are suited to the job.