Washington has for some years been one of the best states. It is a large state with many post offices and a fairly good range of available material. More collectors collect the entire state than focus on specific areas. Whatcom county is the one areas with especially strong demand. However, Seattle and its environs are avoided by most of the collectors.
Territorial material is in the most demand and is quite scarce after the half dozen largest cities.
There are signs that the bull market in Washington covers has slowed. Fewer people are starting collections. The existing collectors are getting more and more advanced, and the market is showing the first signs of being topped out. The demand for better items drags up the prices for everything which discourages new collectors. The existing group still will pay astronomical prices for the scarce and rare items they need, but the low end of the market is becoming unsalable.