Since GPS satellites need to face the Earth, they might execute momentum-unloading from time to time using pairs thrusters so as to produce torque but minimal change in the orbit, but I am guessing that the perturbations are so low in MEO and the way the system works mathematically is such that they would never have to make orbital maneuvers once deployed.
Is that correct? Or do they from time-to-time do some kind of station-keeping in order to remain well-spaced and distributed within each of their orbital planes and slots? If they do, wouldn't they have to deactivate the satellite's GPS coverage until a new ephemeris could be determined and uploaded?
(see this comment)