My recommendation, follow your coach’s guidance.  Personally, I believe the hook grip should not be released. I feel teaching someone to release the thumbs can be unsafe, and it adds one more element to the lift,  complicating things further. Even though the hook grip is no longer needed in the overhead position, it certainly does not hurt the athlete nor place them at any disadvantage. I teach my athletes to use the hook grip throughout the lift in an effort to keep things simple and without additional work.