One of the guys I work with said his mother used to mow the cannas over with the lawn mower. She'd pull them and store without washing them off. I would be more owrried if you pulled them from outside too late. I live in SE Wisconsin which is also considered zone 5. I was really busy in Fall and we had a late frost. I had things which still needed to be pulled that I hadn't gotten out of the ground. I ended up getting them out after several snow falls and they looked fine. I think that if the ground would have been frozen it would have been a different story. Although this would give you an excuse to buy more cannas! I would only purchase varieties you don't already have. If those tubers are still firm you should be okay.

As long as they don't get soft, black, and mushy, they should be fine!