I am no nail expert but a thought that often passes through my mind is that generally the grain direction of one piece being nailed to another is at 90 degrees e.g. in the case of floor boards being nailed to joists. That being the case the nail has to be able to perform both with and across the grain so what would be the preferred nail orientation when nailing down floor boards ?
I would say preferred nail orientation would go with the floor board's grain. The joist is the more substantial timber and could resist cross-grain interference better than the thinner floor board... I'd say orient a cut nail parallel with the floor board.
(There. I think I'm done posting on this topic for the day!!)