I've cut mine back both in December and in the spring and I didn't see much difference. The December prune followed a lot of storm damage. Actually i've taken to giving them a pretty good hair cut in May too so that they are bushier with more flowers.

Being as far north as Z5, I doubt they will break dormancy if you prune them back now.

If you are pruning now (in mid-winter) I wouldn't prune back lower than 2 feet or so above the ground. Buddleias are borderline hardy in zone 4-5 and pruning is best left until you can see the new growth in the spring.