As a fully approved Instructor, and training company owner, the subject of handbrake /neutral, before operating any hydraulic control should be considered as to the type of conditions and type of truck in use. With rough terrain and tele handlers, this is totally impracticable as more damage is done to materials than if they are allowed to use hydraulics and move to collect and deliver loads. Other types of truck eg: counterbalance /reach, then yes they should be applied to prevent truck movement during stacking/ destacking operations, please note the word should, as you are aware to pass the test and reach the required standard this is a must, otherwise no pass. But many operators have been using trucks for 20 years plus and never had an accident, and probably never use either, so does this make them a bad operator:: NO it is all down to experience which there is no substitute for, also with many modern trucks braking is automayic anyway.
All accidents are operator error mainly due to to the old addage, familierarity breeds contempt.