Generally speaking, rhododendrons do not need a lot of pruning. Deadhead spent flowers and remove dead, damaged or diseased wood. If additional pruning is required, it is all about timing. Buds for next years flowers are set in the fall so pruning at that time will more or less destroy next years flowers. The best time to prune is right after the flowers have finished blooming. Prune lightly every year to maintain size and shape but don't be afraid to prune established plants hard. They will usually recover within a year or two although you will be sacrificing next years flowers.