I cut mine down a week or so ago, and already they are showing signs of growing again: having done it once, I'm leaving them - I'm sure the frost will do the work a second time, but in the meantime, all the tall, untidy stuff has gone.

It also stops too much self seeding.

I wouldn't mind a bit of self-seeding, Volunteer, but none of mine actually flowered - this is their first year: I only sowed them this spring.

I love to see them blowing in the wind so i leave mine and I also leave them in case the birds like them.

I'm with Grandmage! Let nature take its course. They might prove invaluable to some insects - i dont know - but I leave them just in case.