severe pruning should wait until fall or winter. Most of the time, cutting back hard to rejuvenate a rhododendron will be okay -- i.e. the plant will resprout. If you're cutting back to stubs with no foliage on them, however, it's safest to cut back only some of the branches. IF they send out new sprouts then you can safely cut the others back. Once in a while, a plant simply dies when you cut it back too hard, so this is a good way to make sure you don't lose the plant.