Joan, I think it's much more to do with HOW you cut back that nice big plant than when - if you cut off the ends of all the branches with the nice healthy green leaves, that will do a LOT more harm than if you just look underneath and take your big strong loppers and cut off one big branch right at the base. The 1/3 rule still applies, so use your judgement. Looks like it's sprawling sideways a lot so don't cut a vertical branch out of the middle, cut off one of the horizontal ones so the plant won't be so wide.
This will reduce the overall size of the plant, but not denude it of foliage it needs to sustain itself. Go for one big branch now, then another one in early spring before it blooms. If it's not down to a manageable size then again cut one more branch in late summer or fall.
It will also look a lot better overall than if you scalp the tops off it. It may have one bald spot for a little while but will most likely fill in.
