Unlike most any other subsystem of any car, the refrigeration portion of the a/c system can actually have its life shortened by preventive maintenance beyond just keeping the fins clean on the condensor and seeing that there is a good and properly adjusted belt in place. The other thing you can do is occasionally put a thermometer in the vent and see that it is up to performance level by following the performance test in the MB service manual.

It is a bad practice to put gauges on the system as a preventive maintenance measure. Every time you do this or in any way crack open the system, you introduce the chance of allowing moisture to contaminate the system. Moisture combines with the refrigerant to form an acid that will eat up the components from the inside out.

As far as misbehaving vents are concerned, it is HIGHLY likely that the cause is a bad vacuum pod. There are about 6 or 8 under the dash that go bad with age. It is a significant job to remove the dash to replace them. If you were to remove the dash to replace one of them, it is HIGHLY recommended that you replace all of them. BTDT.

Good luck,