it really depends, tbh

what faction, what playstyle, the size of thr city, etc...

as above, i prefer to use factional troops when necessary, and use only 2 or 3 cities as main troop centres per region... That way, when i've long finished assimilating that region, i can work on boosting population, and not have to regret wasting money for troops i no longer need... Not to mention, not all troops are good in each situation