Jim, You can leave them in the ground until the ground freezes. A heavy frost or a few nights in the 20's won't hurt them usually as long as ground doesn't freeze. If you are concerned or delayed in getting them out of the ground just put some mulch over them but don't delay too long in your climate.
Here in the Atlanta area we can leave them in the ground all winter with about 6" of mulch over them as the ground never freezes. The temps usually get down to 12-15 degrees for the low for winter but one year got down to -8 below zero. I had left a few tubers in the ground that year and most of them didn't freeze. Some did though.

I talked with a man from central Ohio who didn't dig his tubers in the winter. He piled leaves about 2' deep over them and they didn't freeze.

Hi Jim
You could leave your tubers in the ground till a hard frost. Usually anywhere from October to Thanksgiving the above ground stems are brown and dying after a light frost or 2, then a hard frost. You should let the frost kill the tops off and cut to about 3-4". The eyes will show better if you've let the tubers ripen and harden off after the frost. That is if you want to cut the tubers up in the fall. Poochella has posted wonderful descriptions and photos in past yrs - just do a search. You can also wait till they start to sprout in spring to find the eyes.

If not you can employ my extremely difficult method - lift the tubers and put in brown paper bags and write the name and color. I don't clean them - I just store them. You can put a few handfuls of peat into the bag. Then I lay all the bags into a box and store in my garage up off the floor. My garage rarely freezes in the open air - so the box remains around 40-50 degrees. If its a cold winter I wrap it in bubble wrap or double box it. I lay in a wheelbarrow and pile tarps on top or something to insulate.
I've had good luck with this very low end technique last winter and didn't split my tubers last yr, so I may have to next yr.

Huey - whoever kept tubers in the ground in any part of OH must live in the city and have a super protected spot. Our ground freezes 6" deep or farther depending on the yr.

I've left tubers in the ground in here in Zone 5 - same latitude as Syracuse and they are mush as soon as the ground starts to freeze.

Poochella also gave me the tip of sprouting tubers in plastic bags in the early spring inside the house. I put a couple handfuls of potting soil, a tablespoon or 2 of water and lay the tuber in the open bag, I lay them out on plastic trays from the Dollar Store in a sunny warm window.