If you use mulch over the winter, don't forget about removing it! If you leave it on too long, light won't reach the already-growing plants, and they will become spindly and weak. The mulch also creates cool, damp conditions that are perfect for diseases. It is especially important to pull it away from tree trunks and woody shrubs, which can rot.
Mulch keeps the ground from warming quickly in the spring. If you are confident in the weather, remove the mulch so the soil can absorb the sun's heat during the day, and reflect it back to the plants at night. There are times, however, when you may wish to leave the mulch on longer. Marginally hardy fruit trees (such as apricots) often bloom too early in our climate, allowing their flowers to be killed by a late frost, and reducing your chance of a harvest. Leaving a thick mulch around the plant (but not touching the trunk) can delay blooming, increasing the chance of seeing flowers and fruit.