No, you do not. Contrary to popular misperception, you don't have to spend your entire Saturday afternoon in the extremely rewarding give-and-take of talking and debating with your fellow community members at the caucus tomorrow. If you're some kind of misanthrope who doesn't enjoy the free exchange of ideas and the feeling of true participatory democracy, you can, in fact, just show up, express your preference, and go on to whatever it is you have to do that's more important/enjoyable.
Here is state Democratic Party chairman Dwight Pelz responding to a question on KUOW recently:
Q: Can I sign in and then leave?
A: At 1:00 we convene. About 1:20 we move to individual precincts and begin sign-in. If want to leave at that point, you can. You state your preference as part of the sign-in process. Your vote will stand through the day.