I usually stick them, by themselves, in a recently conquered city near the front lines, and let them reduce unrest, increase growth and help production.
When the enemy armies come to retake the city, guess what's waiting a few tiles down the road from the governor? A nice balanced army. In one of my play-throughs, I had Janusk/Tearny reach levels 9/11 by holding Yithril/Magnar off, while mostly ending their turns in cities near their respective borders of my four front war.
