I don't really go out of my way to get certain traits for my generals/governors, but they often gain particular virtues simply because of how I play the game. Because I'm a turtler and I like to focus on internal development, a good number of my guys usually get the Builder and/or Steward traits. Also, since a good number of my governors are veterans of Crusades & Jihads, quite a few of them tend to have the Pious/Zealous traits as well.

As for the more military-oriented traits, I usually don't go for anything specific. I tend to roleplay my generals instead, depending on what traits they start out with. If a general starts with traits that increase his dread (such as Killer Instinct), then I'm more likely to play him out as a ruthless character who will execute most -- if not all -- of any prisoners captured in battle. On the flip side, I usually make sure a general with traits such as Chivalrous or Honourable Warrior behaves accordingly (releasing prisoners and whatnot).