Hi LAnn, I cant keep dahlias in the ground here- too many slugs etc- so I always grow mine in big pots.
For over winter storage I wait until the foliage is blackened most times, or by the end of Oct for me re the frosts. Then I put the pot, on it's 'saucer', into my part glazed porch to dry off for about a week. Then I bubble wrap the pot, cut back the foliage, give it a 'cap' of fleece & shove it under a plant table. So mine are actually stored in their compost, but kept very dry.There it stays until late Feb/early March when I investigate the tubers & restart them on a windowsill inside.
I did try storing them 'properly' ie dry tubers in sand etc one yr in my shed & lost the lot. So back to my usual way.
The cellar sounds good as it wont freeze, it's dry too. Sheds arent frost free & an unheated greenhouse up Nrth will be too cold.
I did manage to keep 2 wrapped/covered pots alive in my big wooden coldframe last winter. They'd been eated to death by the slugs so I'd removed the labels-twit- & only decided to store them when I spotted fresh growth starting very late in the season. Am well pleased as they turned out to be 'good' ones I had & not Aldi's best!
Good luck, J.