This is a problem even NASA has to work with (although technically for different reasons). IRL, spacecraft orbits are perturbed by Earth's tidal forces, gravitational interaction with the Moon and the Sun, and at lower altitudes, a very small amount of atmosphere.
According to the always reliable Wikipedia, geostationary satellites require about 45m/s of delta-v per year in maintenance. Some of the periodic burns that are done are on the order of mm/s, which is an amount of control we do not yet have.
In KSP you do not have any of those exact problems, but it could be taken as an unintended realism boost.