Not until after they bloom. A common mistake among homeowners is to go out in the early spring itchy to do work in the yard and to start pruning shrubs. If you prune spring blooming shrubs at this time—those that bloom on the previous year’s wood—you will be cutting off the flower buds and lose those blossoms for this year. Spring blooming shrubs that should not be pruned at this time include, but are not limited to, lilacs, forsythia, serviceberry, viburnums, and some spirea species. These shrubs should be pruned soon after blooming, but before they set flower buds for the following year. Before you prune any shrub, you should know what it is and when it blooms.
The following websites provide good lists of spring flowering shrubs as well as other pruning information:
Pruning Landscape Shrubs (pdf), University of Kentucky Cooperative Extension
A Guide to Successful Pruning, Shrub Pruning Calendar, Virginia Cooperative Extension
Best Management Practices for Pruning Landscape Trees, Shrubs and Groundcovers, University of Tennesee
Pruning Ornamental Trees & Shrubs, Purdue University Cooperative Extension