You can actually prune them at any time, it will just reduce the number of blooms you have if you do it during the growing season.
Waiting until the leaves are gone is, as you said, easier and that way you can have a better look at how they're growing and be able to shape them easier.
I have heard, although others may correct me if need be, that you can cut them back by 1/3 in a single trimming.
Barb

That's correct Barb. Just been reading up on the subject for some indoor ones and that's the rule of thumb for most plants. Timing is between February and August. If you still want to maintain blooms you can cut every other branch and do the missed ones the following year. Pruning will envigorate the plant as blooms are more prevalent on younger branches.

Rolande in Ottawa Canada

I have a bush that is about 6 feet tall and has no buds on it. How do I prune? Cut the vertical branches? then cut the balance back by 1/3?