I wear my goggles under my helmet for 3 reasons.
1. I can take my helmet off on the lift without having to take my goggles off.
2. Wearing your goggles over your helmet stretches out the goggle strap (over time and especially when the strap gets wet)
3. I hate ear pads on helmets and i think its stupid to wear goggles over your helmet without ear pads.
Under my helmet i normally wear a burglar mask or a buff with a beenie