Topping trees may give you the immediate satisfaction of having a shorter tree, but the reactionary growth will catch back up to the original height sooner than you think. This becomes labor intensive and expensive to keep topping the trees. It also creates poor branch attachments, defects, decay and possibly disease or pest damage, all increasing the chance of having a major problem. That is assuming the tree survives having so much of its crown removed at the initial cut. Consult with an arborist before you make this decision and alter the life of your tree forever.