well I try to keep one or two professional units, a bit of cavalry, and one or two family members in each major region. It's convenient if there are rebellions or some evil enemy tries to bribe your settlements (like the romani did in my current Makedonia campaign)... Plague is annoying, rebellion is bothersome, but nothing, absolutely nothing, makes me more angry than when those enemy diplomats steal my settlements AND my troops! Luckily it seems like I as usual managed to put together a small army strong enough to retake the two settlements I lost, but often they bribe the settlements while they're in the middle of some 20 turns or so long building project... Anyway in this particular campaign it was in a way good because it feels more historical if I'm forced to keep some kind of garrison, and when I convinced myself to pretend the bribed cities were a Getai rebellion financed by the Romani, it felt a lot more logical.