Most mature flowering shrubs do benefit from being pruned every year of two. The most important thing to remember about pruning shrubs is not to fight Mother nature. Pruning should enhance the natural shape of the shrub.
For maximum flower production on spring-flowering shrubs, prune them as soon as they have finished flowering. Summer and autumn flowering shrubs should be pruned first thing in the spring when their new growth starts.
Try to avoid pruning any flowering shrub in late summer or fall. This can start new growth which can get damaged by cold temperatures.