I grow quite a few Dahlias and have decided this year to leave them out. In Pittsburgh the snow cover will not harm your Dahlias as much as the rain and soaking, and in my experience it is the rot caused by soaking that does the damage.
My Dahlia beds are set up to drain, but if you leave the plants out you have to watch that you block the shortened stems when you cut the plant back, as each of these is a conduit for water to the centre, and while you will prevent the freeze from damaging the plants if you put enough compost on top, I think I will also put down a plastic hat or waterproof membrane over each crown on top of dry compost ( with some slug pellets), and pin the plastic hats down with pegs.
Six inches of local compost, plastic hats and pegs will stop the water, hopefully stop the frost, and the slugs.
I will let you know how this works. I have not been diligent enough to keep the tubers well in previous years when I lifted them, though I let about thirty I did lift last year get very dry, and almost to a plant they all came back in gangbusters fashion.
Here is a photo of Dahlia "Ruby".
(Maybe lift some, leave some in, store some different ways, protect some different ways is the route forward).
