There are many reasons to trim trees, and just about as many ways to do it. What is important is that it is done right. The best way to ensure your trees are properly cared for is by only trusting your trees to Certified Arborists. They have verifiable years of experience, atleast a basic understanding of trees, tree biology and tree bio mechanics, and have signed a code of ethics clause guaranteeing your work is done to ANSI A-300 Standards