Pruning is done to maintain plants at a certain height in the landscape. Try to only remove 1/3 of the plant mass at any given pruning. Pruning more than this could put the plant into distress.
As contra intuitive as it may sound, you need to trim plants to keep density. If you have plants as a screen for example, you should not let them just grow, grow without trimming. Light trimming the tips of all the branches (not JUST the top!) will create a “flush” of new growth which is where you create density.
The pruning of non-spring flowering trees may be done now- such as oak trees. Most cold hardy trees and shrubs may be trimmed at any time but trees do better when they are trimmed while they are dormant. Flowering trees and deciduous fruit should not be pruned until the threat of the worst freezing has passed (mid to end of February). Prune after cold has passed but before spring flowers.
Evergreen shrubs can be pruned anytime during the growing season, March to October. If you prune too much during the winter, a new flush of leaves could be burned by a frost.
Crape Myrtles bloom on new growth so they can be pruned in February.
Correct pruning cuts: see diagram below- always leave a “collar” on the tree and prune on an angle as seen below.