Always wear a helmet! Even if you are in very deep powder there can be unexpected obstacles if you do take a tumble (rocks, etc). People who do not do winter sports have this misguided thought that snow is not hard. Any base layer of snow is somewhere between clay court hard and cement hard depending on temps and water content.

Helmets also give a degree of protection from tree branches when riding in woods or glades.

And remember - nobody is perfect - edges catch, visual/audio distractions - and snow conditions can change unexpectedly. Not to mention there is always the risk of being hit by someone else.