Answer: It is easier to snap the flower trusses off if they are slightly dry, but you can go ahead and remove them if they are still green but the flower petals are spent. The buds for next year's flowers and leaves will be found at the same level as the base of the truss and opposite of each other. When snapping each flower truss off, pull it back sharply and it should come off cleanly without harming the developing buds.
Best wishes with your rhododendrons!
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