This is a good, yet difficult question to answer. We recommend consulting with one of our professionals before doing any pruning as pruning for view, esthetics or for health will often change the way pruning is approached.
There are many good reasons to prune in the dormant period in late fall and winter; first, this provides the arborist a good opportunity to evaluate the trees structure which allows the arborist to decide what pruning action should take place to benifit the tree. Secondly, pruning wounds close faster and with reduced insect and disease in the winter, less chance for the tree to become infected. Plus, perennials are dormant; the ground is frozen allowing heavy equipment to move across lawns and therefore fewer interruptions which saves time and money.
During the summer months is another good time for pruning as trees are less susceptible to losing energy at this time.