I'm no expert, but I did a little bit of research into this a while ago. I read that it is indeed better to make the datum be the opposite surface because they will sit the part on that surface during inspection to it makes aligning to that datum easy. However for blind holes it might not make sense to have the opposite side be a datum because any perpendicularity control should be to the side the hole is on. So when I use positional tolerances I use the opposite surface as the primary datum for through holes and the hole side for blind holes.
Part function and what is critical is the factor, there is no blanket answer.