As elkken said, it is legal to stump shoot in WA, there are no regulations against it. I work for WA Dept. of Fish and Wildlife as a Habitat Biologist, but was a game warden in WA and WY for over 15 years. (Moved and changed jobs due to a family situation). Since I have been a biologist for the last 15 years, I talked to a game warden friend of mine before adding to this thread. He is an excellent officer and very fair. We both had the same ideas on this.
A couple things to remember and think about. What would a reasonable person think you are doing? This is the basis I always used in dealing with situations. If they are not familiar with traditional bowhunters, can you articulate what you are doing in such a way as to ease any possible suspicion? Does your gear support what you are doing? In other words, for example, stumping with mostly broadheads in your quiver would raise suspicion. Remember, game wardens are human too. There are good ones and a few not so good ones. Most are reasonable, especially if the person that they are contacting is reasonable.
It would be wise to have your hunting license with you if there is a small game season open.
All species not listed in the hunting regulations ARE NOT fair game. Actually most are protected. You can hunt fox squirrels year round because they are not native wildlife in WA, but many of the ground squirrels are protected. It would be wise to call your regional WDFW office and get a list of protected wildlife, and also to contact your local game warden and talk to him or her about stumping, so they understand what you are doing.
With all these words, guess I can't claim just my 3 cents worth, but maybe a dimes worth.