Cheaptard automatically switches your equipped ammo at certain circumstances. I made it since I often forget to switch to bad arrows on trash and good arrows on bosses.
To configure Cheaptard, type /cheaptard (or just /tard). Alternatively, open Blizzard's Interface configuration window and go to Addons.
Here you can set Cheaptard to automatically switch to your best or worst ammo for duels, arenas, raids, boss encounters, and battlegrounds. You can also set it to switch to worst ammo when leaving combat. Personally I have all except raids checked.
The addon also adds two keybindings that let you manually switch to your best or worst ammo with keys. Set these keybindings in the normal Blizzard interface.
The way "best" and "worst" ammo is determined is by level first, then rarity (as in epic, rare, etc).
Notes:
- This is my first addon, so it's likely there are bugs. It has been tested in raid environments though.
- Switching ammo on duels intitiated by yourself is not working too well; it will only switch when you enter combat and are targetting your opponent.