Well, it all depends what time(s) of year you are planning on going. Storms can come out of nowhere in both the Olympics and the Cascades. Due to the gap in weather coverage because of the Olympic peninsula, weather reports cannot be considered reliable for more than a couple of days out.
I'm a big poncho fan, but since I've moved to backpacking in the PNW (as opposed to Arkansas) I've moved to taking a dedicated rain/wind jacket. Rain and wind can come up strong and last a long while; the poncho flapping around can get annoying.
Of course, nothing is more important than experience in the backcountry, so only you can make the right call for you for a specific trip. I would just, in general, be prepared for a 40F degree difference between day and night if you get any elevation in your trip.
Best of luck, you'll have a blast! It is absolutely beautiful up here :)