Rhododendrons can benefit from pruning but I find the idea of power shearing rhododendrons off-putting. Yes, power shears are fast but rhododendrons aren't hedges. It's impossible to shear the shrub without shredding some of the foliage and leaving stubs.
In the picture above, I had to hand snip the rhododendron anyway to clean it up. I couldn't just walk away.
I suggest taking the extra time and pruning your rhododendrons by hand. Carefully. You can pinch off spent flowers and cut away last year's growth if height is a concern. Keep your power shears for cedar and boxwood hedges.
Vas Sladek, B.Sc., CLHT