The California Supreme Court unanimously holds today that people found not guilty by reason of insanity have a statutory right to not testify at subsequent proceedings in which the state wants to continue their commitment to a state hospital on the grounds that the patient's continuing mental disorder represents a "substantial danger" to others. Since the statute says that the patient has all rights at that proceeding that would be available to a criminal defendant, that includes the right not to testify.
As a practical matter, however, the state's clearly going to have someone -- probably many people -- testify that the patient is still crazy and represents a danger to others. At which point the patient is going to have to try to rebut that testimony. That's going to be tough if the patient doesn't testify. Especially since my strong, strong sense is that, regardless of what the law is, the factfinder is going to hold it against the patient if they do not get up on the stand and demonstrate some passive lucidity.
In short, if they say you're still crazy, you've got a right, but you'd be crazy to invoke it.