Pruning helps direct plant growth to where you want it. It helps plants grow taller, wide and stronger, ti can also prevent awkward stems and branches from getting out of control while inhibiting disease.
When you are dealing with a hedge you want to time the pruning process with the natural growth surge in late spring, midsummer and early fall. Stop the process when winter is at least six weeks away to avoid damaging new growth by the cold, in this way you will avoid brown tips throughout the winter.
Spring blooming plants generally develop their buds in the fall when they become dormant. You should not prune until flowers fade in the late spring to early summer so the plant has time to develop.
Early summer bloomers including roses often benefit form a light fall prune to thin the branches. Mid and late summer bloomers are prunable anytime they become dormant and often benefit from a thorough prune in the late winter to early spring, which will lead to a heavy set of flowers.