My Arrow Rules:
1. Give it to BLM if they are doing their Solo Rotation. Never give it to them for their AoE Rotation, this is where they get thrown off.
2. Melee. Especially MNK and NIN. It helps them get through their weapon combos quicker. Watch their TP though because Arrow can help drain it more.
3. MCH. Especially during Wildfire.
4. BRD. Helps with their cast times
5. WAR. Helps push their DPS more
6. Other Tanks. Helps them go through their Aggro-generating combos quicker if they are having aggro problems.
7. Myself and the other healer.
8. SMN. I read somewhere Arrow wasn't potent enough to make their Ruin III casts during Dreadwyrm Stance fast enough to add one more cast.