Answer: In my experience the best time and method is to cut individual branches back to a main stem in early spring. In other words, don't give it a light shearing all over the tips of the branches. Instead, trace the branch back to a main trunk and cut it there. (It will look awful at first but it will grow back! See below!)
Since this will cut off blooming buds along with the stems, I tend to delay until immediately after bloom -- the next best time.
If you have to remove more than about a third of the plant, do be sure it receives adequate moisture and a nice thick mulch during the summer following the "treatment". The established root system should enable the plant to recover quickly and become even "bushier" than before but it will take a bit of time for it to reach the problem stature yet again.
Good luck with your pruning!
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