Pruning conifer trees and shrubs for health should be done anytime a problem pops up and can be safely corrected. Removing branches that have experienced winter damage, or branches that cross and could potentially rub against each other is important because these injuries can be an entry point for disease. Limbs that appear weak because of poor attachment angles (also called a narrow crotch) should also be pruned away. As the tree grows, excess bark accumulates in the crotch. This results in weakness because, over time, more bark and less wood holds the branch to the trunk.
In this video, Bert Cregg, an associate professor in the horticulture and forestry departments at Michigan State University, demonstrates how to prune evergreen conifers (including arborvitae, junipers, chamaecyparis, yews, pines, hemlocks, firs, douglas firs, and spruces) for health.