Hi Natalie! I'm over in Commerce. I am told that spring is the best time to trim it back, that is before the leaves come out. You can actually trim it up until the middle of August. After that, there wouldn't be time to establish new growth before the cold comes. Good luck.
Cookie

I haven't had to prune mine routinely yet because a frightened deer (best guess) leaped into the middle of it in its first winter and I had to tidy up some major breakage then. Like Cookie I like to do as much trimming as I can in late winter. However, pruning shortly after bloom would accomplish both a tidy-up if you wished and also allow the next year's flower buds to form on the new growth afterward. To me the bloom clusters become somewhat unsightly with age, and next year I expect to need to get out the extra-long-handled clippers to trim them off fairly shortly after bloom anyway.

I find that if I prune after the flowers fade in the spring I get a second flush of flowers in late summer. It's not as impressive as the first flush, but it looks great as the leaves begin to change to the red fall color.

I didn't prune this past spring and I didn't have one bloom in late summer. Normally, my bushes are fully covered in the spring and about half covered in the fall.

Thank you all for the great advice.
Happy Thanksgiving!
~Natalie

I have a chinese snowball. It was suppose to be a bush or so the card said.
Anyway, mine is looking like it is trying to bloom now there are buds all over it.
When I trim mine it is after the last bloom time in the winter.
That may or may not be the wrong time, I'm not sure.
But I know it blooms alot through out the year for me.