There is no question when reviewing the literature there are some hand grip and postural positions that are statistically advantageous to building strength or displaying strength. Yet, even though the research identifies the best way to grasp an object there are individuals in any population that demonstrate deviations well outside the anatomical norm that affect body positioning for ideal gripping. The differences in anatomy that affect grip include supination and pronation of the forearms, finger length related to hand size, forearm length, the objects shape related to one's structure, posture and other considerations.
Published in Archives of Physical Medicine and Rehabilitation, “Effect of self-selected handgrip position on maximal handgrip strength.”, the study found that whether sitting or standing -- “it may be useful to introduce self-selection of the handgrip position in protocols to assess the maximal handgrip strength.”
Sometimes some things do not require a great deal of coaching and are best left up to the individual to Get Strong.