Related to this, pin tumbler locks on doors should be installed so that the bitting (i.e., the teeth on the key) face up when inserting the key. If you follow a standard orientation, you don't have to think about which way to orient the key when inserting it, especially in the dark.

There's a technical reason why "bitting up" (teeth up) should be the standard way to install pin tumbler locks. If the bitting faces up, the pins in the lock are directly above the bitting, and the springs are above the pins and not being compressed by the weight of the pins. If the lock is installed upside down such that the key goes in with bitting facing down, then the pins are sitting on top of the springs and may compress down over a period of years. A fatigued spring might not raise the pins to the shear line (the level needed for the lock cylinder to turn) and you'll be locked out.

It seems that most door installers and handymen don't follow any convention about up or down when installing locks.