As is often the case, the answer is "it depends." In this case, it depends on:
1) When the condo was organized. Washington law says that all condo associations organized after July 1, 1990, "shall be organized as a profit or nonprofit corporation." Since incorporation is a legal requirement for these condos, the Board must file for incorporation, regardless of whether members approve.
NOTE: Washington law does not impose any penalty on condo associations that do not incorporate.
2) What your By-laws say. As I recall, before July 1, 1990, Washington law did not require condo associations to incorporate. If I am right, the Association does not have to file for incorporation. Whether the Board has to get approval from the members before filing for incorporation depends on your Association's By-laws. The By-laws tell who has the power to make which decisions.
3) It may be worth it for you to talk to an attorney who specializes in condo law about your condo association. Washington law imposes many requirements on HOAs, and many HOAs ignore these requirements. An attorney can give you practical advice about bringing your condo into compliance with these requirements.