Depends on whether or not you find the sight depressing. Once the foliage has died, the plant will depend upon its roots for life. Personally, I prefer to cut the dead stems back since I don't find any winter interest in their stems - which usually smoosh-flop after a hard freeze. Mirabilis makes a tuber-type root and will usually come back up next spring - along with lots of little babies sprouting from seed. I put a label/marker near the plants that performed the best, and ruthlessly pull all the rest.