If i'm really trying to pound an enemy into the ground and already have a bustling, productive empire, I will usually raze the larger cities so I don't have to waste my offensive units garrissoning them. Sometimes i'll keep smaller cities if my troops need some healing, then maybe abandon it when they're back up to snuff. Its easier for me to wipe out my enemy or seriously wound them, then move in later with settlers. Sometimes i'll bomb the city til there's hardly any pop left, or if they draft their pop below six and the city has some good improvements, I'll keep it. my 2 cents.