Yes, cut them down. They are finished for this year and the butterflies are gone. If you want the seeds to fly all around to make new plants, then leave the plants till the pods open. At this point you're only attracting those milkweed bugs (harmless, though) and aphids. Next year, new stems will come up from the roots.

I would cut them down to. If there is still a lot of leaves on the stems I would wait till the leaves fall off.