That may make pretty grass, but it can be harmful to your tree.
The soil beneath and around your tree is where many of its roots are located. Tilling or using a Harley rake around trees can damage those roots and negatively affect the health and stability of the tree.
Before having anyone work around your trees, call an arborist. Understanding where the roots are and how to protect them can help prevent unnecessary root damage and preserve your trees for years to come.