OP, although I now live in SC, I'm originally from Washington State, so can be of some help to you I believe. If you're just going up I-5 to the Puget Sound area, you most likely won't need chains. However if you're going from the west side to the east side of the state it would behoove you to carry chains if you're taking US 2 over Stevens Pass or taking I-90 over Snoqualmie Pass as these 2 passes often have winter conditions where AWD or chains are required. White Pass is less likely to have chains or AWD requirements, but at the very minimum traction tires are often required. The 4th west to east route is through the Columbia Gorge where snowfall usually isn't a problem but can have the occasional ice storm. The Washington State DOT website has a pass report that you might find handy. Cable chains would be my choice with a FWD Terrain, they work better with radial tires and are easier to mount.