I was in OR until 2004 for 7 years (zone 8). I grew Cannas in the ground and left them there year round. There was no issue. They are hardy for that zone. Even in zone 6, I see few cannas, but I have seen them left in the ground and they come back. I think ours is now zone 7 effectively for last few years.

As far as cutting it down, you dont have to do anything. Canns foliage will die in Oct-Nov timeframe. At that time you can bring the pot in the garage or something to protect it from wind and cooler temps(keep it in dark area). The plant will then go dormant. When spring comes, you can bring pot to where there is some light. As new growth comes, you cna then start watering. Initially Cannas will not need water as there is no foliage. If you can, put it in the ground now. You avoid the overwintering stuff.

Ajay

All my cannas are now in pots on my covered deck. They sit near the edge of the deck so they get enough sun to grow and bloom. I leave them in the pots, on the deck, year round but I move a few of them closer to the house during the winter so they don't get wet from the rain. I really do it more to protect my pots from freezing than the canna! :)

They stay dormant all winter and when it starts to warm in early summer they appear.

novicegardener, it is rare for canna to survive zone 6 in the the ground come winter. maybe if it is heavily mulched, or in a favorable micro climate it could. i would not trust it with the ones you prize.

I'm in London, Ontario, Canada. Considered zone 6.5
I was given rhizomes last October that had just been split and given explicit instruction to keep in peat moss in cool dark place. They never made it into being stored in the peat moss, but were kept in a plastic grocery bag in the dark in my basement over the winter. I planted at the beginning of 2nd week of May along back fence. It was too hot without the wind and they started to rot. I moved them along chain fence in fairly windy area. They love it there and only the first 2 sets of leaves showed any sign of slight shrededness from wind - all other leaves are fine and are now over 6' tall and got first seeds today. Each rhizome had 3 or 4 shoots on it. I did not trim the dried material when I planted, but let it disolve naturally. There are now anywhere between 18 to 24 stalks coming out of each one after one season. In this zone, I definitely intend to treat them like glads and pull them out in October. I will likely leave about 4" of stalk attached. I would not advise mounding like we do with roses for winter. The rotting issue I mentioned earlier would likely happen with the January thaw if I was to leave them in and then get 2 or 3 more months of snow. I've realized how sensitive they are and would consider potted canas to be even more sensitive in this zone if left outdoors. I'm pulling mine out of the ground.