I cut mine back after they drop their leaves in the fall. You might not get flowers the following spring, but they should bloom the next year. You can pot the cuttings for new plants.

In your zone, I would wait until February to cut them back. If you cut them back now, it will spur new growth that will die if we have a freeze. Remember, they have to branch into three branches to bloom, so if you cut off all the branches, it will take awhile for them to bloom again.

C9, I would also wait until early spring. No sense doing it now unless you need to cover or protect the trees. Some cultivars (mostly rainbows in my experience) have die back issues from a cut in late fall or early winter. There is some good information on this forum about pruning techniques that may also be of assistance to you. Plumeria branches will generate from the leaf node scars nearest the cut. It's total luck of the draw how many branches you may get. I try to make my cuts along an axis that near as many leaf node scars a possible. I also use a pruning saw rather than shears.