Heavy growth can cause trouble
Moss can break away into gutters and valleys, holding back water around outlets. Thick growth also hides cracked or displaced coverings.
Pressure washing can be rough
High pressure can force water beneath laps and damage the surface of weathered concrete tiles. Old slate roofs can also be damaged by careless foot traffic during cleaning.
Clean only for a reason
If drainage is repeatedly blocking, controlled moss removal may help. A roof does not need to look scrubbed clean simply for appearance.
Inspect while access is available
If safe access is already being arranged, use it to look at gutters, valleys, ridges and obvious repairs. That is more useful than cleaning alone.