Trim any time.

The plantain lilies will soon evaporate on their own, I would not bother messing with them now. Peonies may benefit from having spent tops cleared up so as to reduce the amount of material around to support disease. But I would wait until after they have produced their autumn color display. If you don't allow that to happen then all they are giving you is a couple weeks of flowers each year.

Be sure to stay away from their winter buds, at or just below the surface of the soil.