From Fine Gardening:
http://www.finegardening.com/how-to/articles/pruning-subshrubs.aspx
How to prune early bloomers
Prune early bloomers in spring after the first flush of growth. The best time to prune subshrubs that bloom in spring and early summer is when leaf buds emerge in spring. Although you may be tempted to prune earlier, it is best to wait until you see a little green before removing the dead tips of branches and old flowers. Live wood can be easily mistaken for dead wood if pruned too early. After a wet winter, it is not unusual for subshrubs like creeping thyme to have many dead branches. Just be careful not to significantly prune into live branches, or you will delay the bloom time and you may sacrifice flowers.
Cut back early bloomers again after flowering. Early-season bloomers can also be pruned again just after their flowers fade. That is when plants should be deadheaded and shaped. Deadheading the numerous flowers on some subshrubs, like lavender, can be tedious if done one at a time. I find it easier to grab a handful of spent blooms and cut them all at once. Any flower-stem nubs left behind are quickly concealed by new growth. If you’d like to shape your plant or bring it back within bounds, you can take care of that while you deadhead by cutting three to five leaf nodes below each flower spike. Just don’t cut your subshrubs back to the ground. Leave at least four to five obvious nodes above the ground where new growth can emerge. Some subshrubs may initiate new growth from the roots, but don’t depend on it.