This is a very emotive subject. My gut instinct is that they can have anything they want when I'm dead but if I lost a cherished child and found they had been reduced to a shell without a by your leave, I am sure I would be devastated.
I really don't think there is an answer which will satisfy everyone, but the opportunity is there to register, before a terrible happening, so that you don't have to make a decision while in the depths of grief