Sure - even if they have started making new growth. Make your cuts just above an outward facing bud, 16" below where you want flowers to appear in May. Pruning back to an outward facing bud results in more sunshine to the interior of the plant and more flowers.
It's prime time, Jane. This article from the News and Observer gives an excellent description of how to go about it:
Also make sure you check for any rubbing canes that can create bare spots and allow diseases into the bush.