Early game, I'd say that
most commanders should stay away from the front line of combat, but there are certainly exceptions. Pangaea's centaur commanders, for example, can easily handle being put close to the front, with orders along the lines of "Hold, Hold, Attack Closest", depending on the enemy. Archers, Militia, light infantry, aren't really a threat to him. Crossbows, heavy cavalry / knights in the province being attacked require moving such commanders to the rear and perhaps removing the "Attack" command altogether.
Primarily, it's the armored cavalry commanders I'll throw into the melee - they usually have enough defense and armor protection to do quite well, and also have the mobility to outpace the enemy when things go sour.
