Five important factors which will influence your decision on where to plant trees in your garden:
Framing views you prefer and at the same time placing groups of trees and tall shrubs to mask less desirable angles
Groups of trees planted on raised berms define both access and where desired functions of the garden will be - e.g. play areas, quiet sitting spots, family spaces, street/garden defined.
Bringing canopies close to sitting spaces and children's play areas to provide shade and cooling.
Anticipating new construction to your home or nearby areas (additions, decks, driveways, etc) and then planting fast growing trees on high berms.
Succession planting so that trees which grow quickly (but often have only short lives) can create the desired effect but will also act nurse trees for more long-lived trees with more desirable features.