As with so many topics, opinions vary. I cut the top growth off when the flowers have finished and the leaves have died to tidy it up, but I live in a relatively warmer part of the country. People in colder areas leave the dead stems til spring and the plants are starting to shoot from the base before cutting the old growth off as it may give some protection against frost. The choice is yours.

I do the latter, cutting back in the spring.

And I usually compost mine! Phil J