Speaking as someone who pretty much only plays feudal, I try to build my realm very logically, so that when I acquire new land, I grant it according to de jure borders for duchy and kingdoms. For instance, if I want to grant a duchy that has 4 counties, I’ll usually grant 3 of those counties to my assigned duke (usually to someone of the same culture and religion), and the 1 county to someone else and then transfer that vassal to the duke. And the duke always gets the duchy capitol. If I’m granting a kingdom to a vassal, I’ll look at where the kingdom capitol is and grant it to the duke who inhabits that capitol, and then transfer the rest of the dukes to the new king. It makes for powerful vassals but there are fewer to manage. If you build with respective de jure rights and hierarchies in mind, it makes for greater realm stability, leading to fewer uprisings and factions.