Will all depend on how weathered the boards are. I like to use Behr wood wash think it was called Fence and Deck cleaner to clean and lighten the wood tannins and open the wood up for paint or stain. Follow the directions spray on, scrub some, wait but don't let it dry, then rinse off . I read pressure washing can force crud or mildew deeper into wood grain. A good house cleaner told me its the chemicals he uses not high pressure that is the key. In fact when he washed my house he did not even use a pressure tip. He used the pressure washer just to boost the hose flow/force.
I am not that good with stain I would rather paint. Stain seems you need to do sections with natural break lines and keep coverage more uniformly wet along the leading edge. Once it dries overlap has different color or depth to it.