I don't think it matters a great deal! I've divided them in Sept here, after 1st frost when the tomato plants are done for but have also divided well after heavy frosts when the foliage is turning brown and dead. Think once the night time temps fall into the low 40s high 30s you're good.
I've never divided Bartzella but did my Singing in the Rain last fall, make sure you have a saw handy! If it's anything like mine you won't be able to use a garden knife or shovel like most herbaceous, they're a lot woodier.

Thank you for clarification