I personally do not divide cannas in the fall. By leaving the canna clumps whole you do not have to deal with the smaller pieces that likely will not survive prolong storage (in ground or otherwise).
In middle Tennessee I would definitely heavily mulch any cannas left outside in the ground. You can lift in the early spring and divide-replant at that time. This is also the best time to discard decayed rhizomes and only plant the healthy ones. Any resulting small eyed pieces stand a much better change to growing well.
Cannas do not like cold wet soils in the winter.

Regards,
Kent

I keep on accidentally dividing the canna bulbs Im digging up, but if possible, dont divide until spring after they come out of storage and into the ground again.
Good luck!
-Alex