You can start them early in pots or flats if you like. Plant out when the soil is 60* & they'll take right off. If you plant when the soil is too cold they'll just sit there & if it gets too wet, they'll rot. (wet?? In W. WA???>:)
You're taking chances by not digging in the fall but you could try mulching really deeply- if you have some nice favorites you should dig them just to be sure they don't get frozen over the winter.

Hi, I am on Vashon Island so probably about the same zone as you, 8b. I have been leaving my dahlias in the ground over winter for 7 year now, with none lost from freezing. I do put a straw mulch over them when the temp is dropping below 25 but just dug up some that I had left in an old garden and not mulched this winter and they were fine. My soil does drain well, however. I loose a lot more if I dig them up!

I am in Silverdale, WA and I have had a Dahlia come back twice on its own without being dug up as well.