Topped trees are not only unattractive, they are also dangerous. The branches that grow back after a tree has been topped will be weakly attached. As these branches grow larger and heavier, they can break and cause damage and injury. Choose trees that have an ultimate natural height lower than the height of the lines.

See:

http://ucanr.org/sites/UC_Master_Gardeners/files/131092.pdf
http://ucanr.org/sites/UC_Master_Gardeners/files/136380.pdf