It is a general rule to prune trees while they are dormant to minimize damage and stimulate new growth.
Fruit bearing trees are pruned during dormancy to reduce crop quantity, but increase the fruits quality. This timing will also aid in new bud formation the following year.
Spring pruning should be complete before the buds begin to open. However diseased or damaged branches should be immediately removed. Evergreens do well if pruned right after the wood has had a chance to thaw. To achieve a compact effect on pine trees, pruning should be put off until late June.
Pruning has a dwarfing effect on all trees but more so if it id done in June and July. Flowering trees should also be pruned during the summer after the trees have flowered. If a tree is particularly late flowering it can be pruned in the spring just before active growth begins.
Trees such as Maple or Birch are considered bleeders and should be pruned in the summer, when the leaves are fully formed. Summer time pruning will minimize excessive sap loss.
Late summer and early fall pruning should be left for dead wood only. Any extra pruning at this time before dormancy can stimulate new growth, this growth will not be able to harden off adequately before winter. Damage usually includes cankers and excessive die back. Deciduous trees thrive with a late winter early spring pruning.