If the plants are already dying back, then yes, cut them down to the soil line.
If they are still green and seem to be growing, then I'd wait and let them finish out the season. Allowing them to die back naturally will ensure that the plant has stored away all the energy it can in the rhizome for next season. Cutting them early will just interrupt that process.
Definitely remove all old plant material after the plant has "died" back. As David_42 states, this prevents nasty bugs/fungi from overwintering on the remnants and reinfecting the new growth next year. This includes removing any fallen leaves from the ground around the plant site.