Burying the containers, or setting them directly on the soil and mulching heavily is a really good way to over-winter, especially since you are keeping them out of the sun. I think this way of protecting the plants is preferable to over-wintering in an unheated garage or out-building. When over-wintering by mulching, your biggest concerns are the roots drying out completely and/or rodents/deer damage. The drying out worries are pretty much eliminated if you bury the entire container, though.

Plants will be almost as cold tolerant as their counterparts in the ground when you bury the containers, and mulched containers only a little less hardy than those buried.

Al

I group all of my pots together in between two of my raised beds, then mulch them in... so far, so good. I'm hoping everybody survived this past winter, but I won't know for certain until it's time to move the mulch and bring the pots out into the open.

There are several methods for wintering over potted plants outdoors... I think the way you describe doing yours is a good one... I've also banked in plants by digging a shallow ditch, laying the pots on their sides in the ditch, and then covering the pots over with soil and mulch, leaving the plants sticking up on an angle out of the ditch. You can bury pots directly in the garden, up to their rims, and then just mulch over as you would any other garden plant...

I'm sure there must be more methods of wintering over container plants... but I opted for the quick method of grouping and mulching this past season, due mostly to the fact that I was up to my eyeballs in bulbs that needed planting and other autumn garden chores!

I grow a lot of perennials in big 5 gallon buckets and usually I group them together like jodik. But since mine are mostly patio plantings I get the bunches as close to foundation walls as I can. I've had things come back well so far.
Peace
George