Keep them all, the ducks don't care if they're landing in same species spreads...that has been my experience in the central flyway of Montana anyway. If you have to pare down for moving, keep your best looking decoys regardless of species and buy some goose decoys once you arrive at your destination...then you're set for whatever flys your way.
Well... I live in Washington, and there is a huge variance depending on what side of the state you are in. On the westside of the cascades there are all of the common ducks (mallards, pintails, greenwing teal, wigeon) plus a mix of other species (ringnecks, goldeneye's, bufflehead) and you have the opportunity to hunt the sea ducks too (scoters, oldsquaws/longtailed, harlequins, mergansers, etc). I would say pintails and wigeon rule the westside, but there are plenty of mallards and teal too. Pretty much everything in North America can be hunted in Wa with the exception of Fulvous ducks and Cinnamon teal (though I know of a couple that have been shot). There is also some good Canada goose hunting to be had as well as a smattering of snows and specks. We even have some limited days of Brant hunting.
On the Eastside of the cascades, Mallards are king, but pintail, teal, gadwall, wigeon are all prevalent. You will see more bluebills, Cans, and Redheads on the eastside as well along the Columbia River. Canada geese are prevelant, might see a few snows and specks as well.
As far as I know- and depending on where you are in each state, ID and MT follow along with Eastern Washington as far as huntable species. You will see more snows and specks in those states as well.