Here is yet another idea to simulate and handle corruption.
The distance factor could be modified by technology. Sort of a 'speed of comunnication/travel' type of thing. In ancient civs, the distance from the capitol to the edge might be measured in hundreds of miles, in modern times, it would be thousands of miles. (I know map size does not relate to actual 'milage'.) For example, in the 'old west', fronteir cities were basicaly on there own, and coruption was common. Now these cites are concidered part of the main culture, and corruption is not as much. (As for the distance factor versus real life, many people consider Washington DC to be the MOST corrupt city in the us... ;-) )
Under this approach, when I certain tech is reached, (say printing press, or radio) communication and control is increased, so the distance factor could be lowered.
Other factors could be: on the same continent, continuous 'culture' link to the capitol, open trade route (no road/harbor=no way to supervise), era (simular to tech, but simplified)