There's no need to dig it up in zone 8 Washington unless you just want to. It comes back reliably for me in SE Oly, one of the cooler spots in western WA for night temps throughout the year.

Nice web page at:
http://www.paghat.com/cosmos.html

Hmmm. I planted one about five years ago, and lost it over its very first winter. It was not as well established as these plants, but I do really want to be sure these make it as they have become a real focal point. My understanding has been that overwintering them here is not at all guaranteed.

Like many zone 8 plants of borderline hardiness, good drainage in our climate is the key. Wet winter soils are more likely to cause its demise than cold. I've had one in a container for a number of years and it comes back reliably. Seems to come on faster and bloom earlier than those planted in the ground as well, most likely as container soils tend to warm faster in spring. And I do nothing special to winter protect the container.

Good point. I have sandy soil - I'm sure the tuber could turn to mush in a cold, poorly drained clay. What a great plant.