[gregpark]

It's usually not the nail working loose but rather the board working loose due to repeated expanding and contracting with temperature swing. Nailing at opposing angles rather than straight in allows the nails to keep the board holding tight. Even with my framing nailer I nail at a slight angle. The shank of the nail then can help hold the board tight instead of just the nail head.

I screw deck boards down at a slight angle too but for additional reason. Rain water follows the deck screw down and the screw thread path eventually rots out in the joist. A slight opposing angle to the screws and the deck boards can't pop up even when the screw thread is not doing its job anymore.