I believe Seattle is quite hilly in areas, correct? If so I'd definitely consider getting winter tires. If the Seattle climate was in a flat Midwestern area you could probably get away with it. Hills are always a game changer IMO when people question whether they should get winter tires or go with AWD over RWD.
You could also justify winter tires based strictly on the temps you'll see. You will be maximizing your traction over all-season tires even when it's above freezing. They say anything under 45F is best suited for winter tires.