I've been told to let the foliage start to die back before cutting them down, but I usually cut mine when I get tired of mowing around them. It has never hurt them.
You should do as Melissa has been told and not as she does! Deadhead the plants after they bloom but leave the foliage until it starts to yellow in the fall.
Douglas is absolutely correct. The rhizomes need the the nutrients from the leaves for next year's bloom.