I prune my buddliea back after I see new growth in the spring. Here it usually dies back to the snow line. But if you wait until you see new growth in the spring, Mary, and cut it back to there, it will still grow to over 6 feet, unless you have the mini ones.
As far as Hydrangea paniculata goes, I prune mine in spring, which normally only consists of cutting the dead flower heads off, but sometimes I cut back a little more. Early spring, when you're starting to clean up the garden. I don't think you can hurt those, though. Prune pretty much anytime.
