You can cut them after flowering if you want it tidy. I tend to leave mine until spring so that the old stems protect the new growth.. they also provide somewhere for critters to over winter.

If you have the very tall ones and want tidyness plus protection you could cut them back halfway...

I must admit that when I'm itching to start the big tidy-up in the spring, I start with the asters and phlox. They are tough and survive every winter.