It doesn't matter if it's zombie-like, as long as it doesn't impede a nice smooth rotation of the torso. At least, that's what I was taught.
I tend to tuck my right (I'm left-handed) elbow behind me during the follow-through. I seem to remember that Agassi (?) did something strange with his right arm on his forehands. In the end it doesn't matter and no-one notices it anyway.