If there's no problem with overcrowding and you have decent health in major buildings, you should keep upgrading their economies to take advantage of their population (if you had a maxed out settlement with every advanced building but only 400 pop, it will earn less than a simple province with level 2 government and buildings but 12000 pop. Higher pop earns more with buildings) However, keep an eye on them to make sure they don't get the plague, losing soldiers, population and therefore money. You can govern all your cities if you choose to 'Manage all Settlements'.