Butterfly bushes are hardy plants that can be pruned in the fall, or any other time of the year for that matter. Generally speaking, it is best to prune them while they are dormant. This is best done in the winter in warmer climates.
There is no absolute answer about the best time of year to prune a butterfly bush. Pruning in the spring can be done, but it's best to make sure there is no threat of frost afterward. After pruning, it's a good idea to add an extra layer of mulch around the bush for insulation and protection in colder climates. There is no need to worry about stressing the plant if it's pruned in the spring or summer because these plants handle stress well and come back stronger than before.
Certain pruning techniques promote vigorous growth and healthy blooms as well as a more manageable plant. Cutting a butterfly bush back to a foot or two from the ground is the best way to keep it from becoming unruly and promote new growth. Trimming or pruning isn't a bad idea throughout the year. Trimming the branches whenever they are unruly helps keep a defined shape and rejuvenate the plant.