I've seen that in some movies. Most of them did it most likely due to the visual special effects. But there was one movie (I can't remember the name) where the archers were first using burning arrows, but then one side used "normal" arrows. The enemy couldn't see the arrows coming and were not able to protect themselves with their shields. This act was deemed scurrilous. Did such a ethical code really exist?
No. Medieval armies did not worry about "fighting fair." Fire arrows weren't all that common to begin with (and pretty much were exclusive to siege contexts in western Europe). They weren't used to let the enemy know arrows were coming, they were used to set things on fire.
A full-scale battle taking place at night would also be a very rare occurrence. Although night raids and ambushes with smaller forces did occur, leading a medieval army was difficult enough without taking away a commander's ability to see.