I used to try to totally relax my arm on my serve, but it didn't work, because my elbow would hyperextend and get hurt.
I now prefer to use a little bit of volitional internal shoulder rotation. It's helped my serve tremendously, and my elbow never flares out and gets injured anymore. I feel like I'm more guiding my arm into the correct rotational whip than forcing it there, but it is volitional, it's not purely relaxed.