Timing is an important factor when pruning plants in the nursery or the landscape. The late dormant season is best for most pruning, with some exceptions. It may be tempting to prune now, but it opens your tree up to disease. Fungi will spread their spores profusely and can easily infect the tree at your pruning points. Pruning in late winter, just before spring growth starts, leaves fresh wounds exposed for only a short length of time before new growth begins the wound sealing process. Another advantage of dormant pruning is that it's easier to make pruning decisions without leaves obscuring plant branch structure.
Here are some pruning tips from Kansas State University Extension's Ward Upham.
Though light pruning and removal of dead wood are fine this time of year, more severe pruning should be left until spring. Consider pruning to be “light” if 10 percent or less of the plant is removed. Dead wood does not count in this calculation. Keep in mind that even light pruning of spring-blooming shrubs such as lilac and forsythia will reduce flowers for next year. Kansas State University Extension normally recommends that spring-bloomers be pruned after flowering.
Shrubs differ in how severely they can be cut back. Junipers do not break bud from within the plant and therefore should be trimmed lightly if you wish to keep the full shape. Overgrown junipers should be removed. On the other hand, there are certain shrubs that can be pruned back severely during the spring. Rejuvenation is the most severe type of pruning and may be used on multi-stem shrubs that have become too large with too many old branches to justify saving the younger canes. All stems are cut back to 3- to 5-inch stubs. This works well for spirea, forsythia, pyracantha, ninebark, Russian almond, little leaf mock orange, shrub roses, and flowering quince. Just remember that spring is the correct time to do this, not now.