Should I vassilize my own sectors after I develop them? Or right after colonization?

Bascily title. My empire is very wide and I already own a lot of planets and a lot are more to go. Is it better to release already developed planets or the ones I just colonized? Also is there a way to specialize my vassals? Like if all I want from them are basic resources and I tax them only it, will they build more of them?