If your yard has the maritime breezes off Puget Sound, then you should be able to plant almost anytime. If unsure, plant in paper-lined pots for sunny days out and cool nights in protection, like Jroot's thread, and transfer/plant outside in May. If you're in a cool microclimate, best to pot up now, and wait until mid May to put outdoors. You don't want them in cold, wet soil.

Here is a link that might be useful: Jroot's threads

Typically, tubers can be planted any time in late April or early May depending on how wet your soil is. If it is too wet to till, you are wasting your time trying to plant the dahlia tubers. Soil needs to be dry enough to till. Dahlia plants need to go in a couple of weeks later as they do not like cool rainy weather and the slugs love dahlia plants.