rationalskepticism.org

I prefer a few big vassals rebelling every now and then compared to countless small ones. It does mean your own powerbase has to be strong though, retinues make a huge difference here, as well as a cash reserve for hiring mercenaries. I've sunk a rather large amount of time into the game though and have little trouble with most rebellions. The harder they are to put down, the better. Landed siblings who are popular with the other vassals tend to cause the most trouble, as a father giving land to an ambitious younger son is almost a guarantee for civil war once your heir takes over

As an emperor vassal kings are great, they have their own vassals to worry about and when they feel like it they can even expand your empire's borders for you. Plus when you raise their army it all appears in one spot instead of having to move all the little armies the dukes can bring to the party.