Pressure washing ANY WOOD is NOT good for the wood. Wooden planks.. have
Soft wood fibers and
Harder wood in any given plank. The fact that you are able to see the
Grain of the wood is showing you the Soft and Harder wood.. The pressure washer removes the soft wood fibers and leaves the harder wood fibers standing BOLD.
YES: It cleans the wood. It will be pretty for a couple of weeks. The weathered (Gray) color will come back in very short order. Now you will have a very rough surface..hills and valleys ..grooves..between the lengthwise grain of the plank..
Whether you stain or not after pressure washing is IRRELEVANT. You will have a rough surface. The SOFT wood is GONE.
NB