IIRC dread gives +5% loyalty per skull with no drawbacks, so it is always good to have. On the other hand, dread-giving V&V's usually come with a loyalty or morale penalty. Soldiers don't like it when you kill off their valuable ransom; and peasants don't like it when they get punished for following their rightfull lord into battle. After all, you owed your duty to your liege lord, not to the king or the country (unles that king happened to be your direct liege lord). The concept of loyalty to a nation only developed during the middle ages.

Acumen for governors gives +10% to the income of the governed province per feather, and +2% to national income in the King.

Personally, I would save a high-dread general for rebellious provinces (Scotland, Portugal and Lithuania are the worst, but there are others). Off course, if you regularly butcher your enemies, you will have plenty of high-dread guys so this is not an issue.