Well, this used to be an easier question to answer. I've lived here in the NW 45 years, and September and October were generally good times for photography, weather wise. It seems things have changed these last years, with more unsettled, cloudy weather during these months.
Summer can be the worst time to visit here for photography on the coast, due to very warm temperatures inland creating a thermal wind (usually a NW wind) that creates upwelling of cold water. The ocean is often colder at the surface in the summer than in winter. There is, generally, less fog than there used to be. Recently we have had more smoke in the summer.
I prefer unsettled weather for photography, and if you do too, late March, April and May can be great for storms and clearing storms, and stretches of clear weather as well. It's always a gamble. There is still snow in the Olympics and on Vancouver Island mountains as well. Mid-October can be good, but recently it seems to be more gray and drizzle.
If you are interested in knowing more, I recommend The Weather of the Pacific Northwest, by Cliff Mass, head of the Meteorology at the University of Washington. His blog is also a great resource. He can be controversial, but always interesting.
