I always keep cities. They are useful to me as long as they get to pop 2 so i can pop rush.
You NEVER pop a settler out if you raze them. At least thats my understanding.
The only time you should raze it is when, you don't have enough units to quell resistance, or you need workers, or you cannot take over that land with your own settelr later.
All this is dependant on what victory your going for
in domination taking cities is incredibly helpful.