Having looked through the posts no one has mentioned the elephant in the room in Washington: horrible I-5 traffic (approximately 110 miles) between Olympia and Smokey Pt. (North of Everett). Used to be I'd advise daytime travel on that route during non-rush (10-2) but often that doesn't help anymore. The best time is the middle of the night. Last time I towed through Seattle I left home at 3am and cruised non-stop at 55 mph. Beware of traffic in the Puget Sound area. It's broken. Outside that 110 mile I-5 / I-405 zone you're pretty much ok.
I can't speak for traffic in the Portland, OR - Vancouver, WA area. I'm not from there.
