It probably makes no difference in the outcome.

For me, it would depend on the surface being washed and the outside temperature. Wetting the surface dilutes the soap. But if it is a hot, sunny day which might cause the soapy water to dry on the surface, then wetting first would build in some extra wait time, which is good because it gives the soap additional time to work. If the surface was super dirty, wetting it down might remove some of the dirt and let the soap work on what's left behind.

A power washer is a heavy-duty machine that is effective at cleaning driveways, walkways, house siding and various other outdoor surfaces. Properly using a power washer is important in maintaining a safe work environment and effectively cleaning your exterior surfaces.
Instructions - Mind it that Move furniture, grills, toys and other objects away from the house to prevent damage. Close all windows and doors. Place duct tape over electrical outlets on the home's exterior. Clean your driveway, sidewalks, concrete patios and walkways in a similar fashion. Remove all objects from the surface you will be power washing. And do not use a power washer on brick, hardboard or wood siding.

graywings - thanks for the reply. Kind of parallels my
thoughts on the matter.

SharperClean - thanks for the reply, but it would have
been nice if you had replied to the question I asked.