Pressure washing is the best way to clean the roof. I would also use a cleanser, such as FLOOD (picture below). Very safe and will do a wonderful job. Safety should be your biggest concern. Make sure you point the force of the spray downward (toward the gutters) to prevent any leaking. Also, don't hold the wand too close to the shakes or you can damage them or cause a lot of weird discoloring. Try to maintain the same distance from the roof all over. About 12-16 inches should be sufficient. Also remember to use the wide fan tip and not the thin stripping tip.
Your maintenance person was most-likely referring to the chemical treatment that is on your shakes. Those need to be redone every 5 years or so. The shakes themselves should last 30-40 years if properly maintained.
Regarding the chemical treatment, there are many different types of treatments and sealants to consider. Careful though, because some will damage copper flashing and can kill vegetation. Make sure the roof is completely dry before you apply anything.