Is there a way to turn off the display of helmets for my companions?
- Open the Character Sheet (C)
- Switch to the tab for your companion (make sure they are active/present)
- Hit the "Customize Appearance" button on the middle right
- Check "Hide Head Slot"
I'm not sure exactly when this was added but it's been at least a few months.
There are dozens of threads on this issue, but to date (3 January 2012) there is no way to hide your companion's Helm.
You are limited to:
- Choosing a helm with limited model (as @Krazer suggests)
- Having your companion not wear a helm