immediately after flowering is done. if you wait much longer you will be cutting next year's blooms. Key is right after flowers are done

Is there any harm in pruning them now before they flower?

If u prune now there will be no blooms this year but won't hurt/kill the plant per se

I second bossyvossy. I'm near Athens and mine are already blooming. Since there's a frost warning for this coming Sunday/Monday night, I'll lose the rest of the blooms to the cold anyhow. So you could proably go ahead and prune regardless. FYI, l had a neighbor prune hers back from 8' to 2' and they have grown back easily over the last several years. About 4' tall now.

Just a warning, you'll probably lose a few. I did this on a dozen in my yard when we moved in, they hadn't been touched in years. About half came back but never really looked right afterwards. Once again, people planting stuff and not considering how big these and other plants will eventually get.

interesting, I gave mine a crew cut last year after they dropped their blooms, trimmed them back to less than half, they looked horrible, this year they bloom as nice as ever, but are a managable size again

I would only cut down one third of the size. You can always cut more off next year after they bloom.