While most pruning work can be done any time of the year without causing any real damage to the tree, winter provides a unique opportunity to really study the structure of the tree. Without any leaves, it is much easier to examine which branches are creating a source of stress on the leaders, and which branches are properly formed.
Normally, most varieties of trees will have one dominant leader that grows vertically and all other branches radiate from. If a tree is not properly pruned or supported at a young age, a tree can develop a co-dominant leader. Pin Oaks and Red Maples are two varieties of trees that are prone to this problem, but we also see the problem with many other tree species.
There are solutions to this problem. The best solution is to simply prune the tree when it is young. Doing so will mean small wounds, and the tree will be able to heal quickly. If the tree is a little larger, a technique called subordination pruning may be used. This is where a portion of the weakest co-dominant leader is pruned back each year. This will allow the tree to slowly adjust to the loss of its canopy and train itself to grow the proper direction. Finally, the tree can be supported using the cable and bracing technique where the co-dominant leaders are essentially roped together in order to support the branches during strong winds.
It is important to meet with a consulting arborist to diagnose the proper pruning techniques for your trees. These experts study thousands of trees a year and understand how each tree and situation will respond to different techniques. If you believe that your tree is in need of major constructive pruning, the winter is a great time to have this done as most pruning schedules fill up during the spring.