I'm in zone eight and i leave them in whatever pot is apropriate for their root size by fall. I usually place mine on my covered porch and water before they are too dry. If they are for your zone should be no problem outside.
Pattie

A general rule of thumb on overwintering container plants is that they need to be hardy to one zone colder than what you are at, since the root ball is more exposed to the cold temps than if the plant was in the ground. Personally, I think their chances are better in the ground. I'd plant them out in the fall, or you could bury the pot in the ground if you don't have a permanent home ready yet.

Bonnie

Token, there is no hurry to get those pots in the ground as their roots are staying in the container and not being established in the ground. It doesn't matter if the plants are actively growing or dormant.

I'm zone 4 and also leave them in a pot appropriate for their root size, some in very small pots, others in larger pots. Late fall, after the tomatoes & other veggies are cleaned up, the pots are buried in my garden bed. They are watered well and later covered with shredded leaves for winter mulch. In the spring the mulch is removed so the plants can get growing and later planted or moved to a larger pot if needed. Check the roots because with spring growth they can get root bound.

Occasionally I will even drop a jug with seedlings into the ground for the winter.

Some of my very large containers are placed against east side of the house and covered with bags of leaves. These are mainly grasses and rudbeckia.

Plants that are not hardy in Z:4 get extra mulch or may be wintered over in my unheated garage, wrapped in paper and stuffed in a box and placed against the wall closest to the main part of the house. This is not always successful but more often than not.

Like Bonnie, I think everything has it's best chance in the ground. But if you bury a pot, make sure it has excellent drainage, especially if your're sinking them into clay soil. Might not be as critical with sandy soil, I wouldn't know.

Karen

Token, here where I'm one zone warmer, I leave a lot of things in 4x5" deep (sometimes larger depending on the plant) pots their first year - wheeled 3-tiered carts on my deck so full exposure. If we have temps into the teens predicted (low teens happen rarely but do happen), I wheel them into my open garage (no door) and throw an old sheet or beach towel over the cart. Last winter was ghastly and I lost codonopsis tubulosa vine (WS 2007) that should have been hardy to Z7 - but I lost the original 5 yr old plant in the ground too. A styrax native to Calif and only hardy to Z8 did fine on my front porch in a larger pot just pushed up against the wall of the house, no damage at all.

I'm a lazy gardener - I brought some pots inside to my basement last winter, stuck them on a table under one of the incandescent lights, which I tried to remember to turn on part of every day. I watered them when I remembered.

Most of what I brought in survived, but I did lose a few.

Butterfly bush is so easy, if I were you, I'd just plant that where you think you want it, color or not. You can always yank it out next year and put it someplace else. The hostas, if they're really small, I would try to winter over - I haven't had very good luck with tiny hostas planted out in the fall reappearing in the spring. But that's just my experience; I believe Tiffy, who is actually a full zone colder than where I was, has done really well with them. But then, I can't remember whether she wintered them over the first year.

Token,
The first winter I had stuff still in pots I covered it all with leaves then put a lattice frame over it to keep the leaves from blowing away-it made a nice home for the meadow mice.
Last year I put all containers in a frame made with leftover decking. it sat uncovered all winter and everything was fine-please note that up here we have snowcover all winter long.
Things seem to do fine if left in containers, for me at least.
-B