The best time to prune Rhododendrons and Azaleas is in the spring right after they bloom. These plants start setting next years flower buds over the summer, so late pruning will cost you some blooms next year, so get them pruned as soon as they finish blooming. It's also a good idea to pick off the spent blooms so the plants don't expel a lot of energy making seeds, unless of course you'd like to grow them from seed. But keep in mind that they don't come true from seed.
Gail x

Unless you want to control the size of the rhodo. you have no need to prune. We rarely prune any of ours. As for removing the spent blooms - life is far too short!