When I first started out I was wondering about this question too. Here's what I've found through personal experience.
Create king titles if -
You want more prestige.
You want de jure claims (in order to invade someone/force them to be your vassal)
You have problems with vassals of your culture (Duke or Count) joining the independence faction
Don't create king titles if -
You want to allow de jure drift to take place - you must be an empire, hold all counties in the kingdom, and not actually own the king title for it to drift into your empire
You have problems with Duke-level vassals NOT of your culture joining the independence faction. Creating the king title will just make them more unhappy, unless you hand out the king title to someone else preferably of your culture.
Personally I usually hold off on creating kingdom titles unless I want to expand, or to discourage a independence revolt (e.g. a few decades after conquering a large area through Holy Wars). The other exception is single-duchy kingdoms, where I will create the kingdom so I can destroy the duchy, then hand out the territories to counts who won't desire your King-level title.
I never, never, never give out king titles to anyone, because Kings will attempt to join the independence faction even if they share your culture, unless their kingdom is de jure part of your Empire. If you're not the HRE or Byzantium, you'll realise there are actually very few kingdoms that de jure fall under your empire, without allowing for de jure drift.
