You're going to have food problems with the northern cities no matter when you take them. In my opinion, it's a good idea to take them all, relatively early, since you will then reach the edge of the map up there, meaning no enemies in that direction. Be sure to not use too large an army, since they will eat the food of each city you conquer, possibly causing it to rebel. 1 or 2 phalangites, 1 archer, 1 or 2 catapults should be enough.
Also be sure to move plenty of sheep up there, as well improving the roads and possibly using workers to transport food.