I don't see any problem with using a pressure washer to clean the nonskid deck. I always use it after I get the boat out of the boat yard as there is always a ton of dust on it from bottom sanding on the boat as well as adjacent boats. Mix in a little rain and you've got a mess. I also typically power wash in the late spring, to rid the nonskid of mildew and dirt that has accumulated over the winter months when I don't scrub the deck routinely. Its just common sense, keep the pressure stream away from deck fittings, hatch frames, chain plates, etc. Its quick, easy, effective, environmentally safe as there are no soap solutions---just water. After the initial power wash in the spring, I usually maintain with Starbrite non skid deck cleaner & a boat brush. I also occasionally use Woodys non skid sealer after the initial cleaning.
I prefer not to use abrasive cleaners like soft scrub as it scratches the gelcoat and makes it dull and easier to stain. I like to keep the cabin sides waxed as well as the cockpit waxed so I don't use strong cleaners like simple green that will strip the wax quickly. Not that I don't like Simple Green--it is a great cleaner. I usually use it when I haul out, to thoroughly clean the top sides before polishing & waxing.