Could be south, I don't think it is always the same, they are just right outside the city walls. It seems to vary, north seems to be predominant to me, they move around. They are different from the units that appear between Ariminium and Tarentum. Typically I don't bribe rebel units till I have a lot of money, I prefer to destroy them. Sometimes I will have armies of pure cav for dealing with rebel armies. That works best if you confine that army to a region, it is quicker to destroy the rebels with units from a city than to wait for the cav army to get there.