I've divided at both times, I like Spring better. As far as the root system, being lazy and impatient, I just hacked at it and I've had no problems w/the divisions growing. Mine were the big ones, I have others that are smaller and more compact - I have never divided those. HTH

They're incredibly easy to divide. Dig around the perimeter and pry them out of the ground, break the crowns apart where they appear "dead". THAT'S IT! Replant them in enriched soil and set the crowns deeper than they were growing. That's it! Water them in well.

You're over-thinking this... just "do it". Astilbes are are remarkably foregiving plants, all they really require is adequate moistiure. REALLY.

I like spring better also. They are smaller, so they take the shock better. I look near the plant and divide when I see a section that looks like it is on its own. I dig down between it and the mother plant, first. This does not disturb the entire plant and has worked well so far. Good to know I could raise the entire plant and "hack" at it if needed. I have some that don't seem to spread as much but are very large. Remember they like water, even more so after a division.

Thanks everyone!
I'll be dividing them this weekend!
Take Care!