Sometimes fall pruning can effect winter hardiness, so when in doubt, I'd go for spring. This site recommends spring pruning for smoke bush, but says they bloom on wood that is a couple of years old, and you may sacrifice some blooms. If the growth has to be at least 2 years old, then you'd be sacrificing blooms in fall or spring anyway.
I don't think last years dead heading of White Profusion would have effected the bloom this year, more likely something to do with the weather. Really though, don't be afraid to prune them aggressively- I just leave a stump. Its best to wait till you see signs of new growth in spring, then cut them back.
