You have confused "evergreen" with BROADLEAFED evergreen. Pines and boxwood are not the same. Treating them the same can lead to problems. However, both can tolerate some things. Doesn't mean they really like it much. If you've found a storage solution that works. Go with it.
The arguments over whether evergreens (Pines and junipers) need light during winter is a long one. I don't profess to know either way with them, but I'd suggest that since these species naturally live outside in the light, that any temporary darkness--whether snow induced or whatever, isn't really a terrific thing for them. It's something they may endure (such covering of snow --in the case of pines---may only inadvertently protect them from winter's worst).
Boxwood, on the other hand, are not alpine species. They are from much warmer climates--some are native to Mediterranean climates. Such climates have ample sunshine year round and temps that don't drop drastically below 0 F. That means, I think, that boxwood require some light (storing any plant where it gets direct sunlight in the winter is a BAD thing) during the winter. I've stored my boxwood bonsai under mulch in the shaded part of the backyard with a windbreak for over 15 years. They begin pushing new growth in late Feb.
Lastly, trees are "evergreen" for a reason. They don't just hold onto their leaves in the winter to be pretty. It is a survival strategy aimed at maximizing their resources. They retain their leaves because the cost of producing a completely new crop in the spring is high. Pines and other evergreens are usualy growing in poorer soils and can't afford the biological costs that deciduous trees can in better soils.
There is also evidence that pines can and do photosynthesize during the winter when its warm enough.