Washing your deck is part of routine maintenance that will greatly extend the life of your deck. Most common pressure treated decks are rated to last 15-20 years. I have had the pleasure of maintaining decks that are double that age due to mindful and diligent maintenance of the structure. It's recommended to, at least, wash your deck yearly, especially if it is in a shady spot in your yard where mildew, algae, and moss have a perfect environment to accumulate. If it's in a sunny spot, then the sun's UV rays can rapidly deteriorate the surface finish of horizontal boards and railings.

Go outside with a spray bottle and spray a few sections of your deck. Does the water bead up or does it appear to soak in? If its the latter, then it may be time for a cleaning and a re-stain