Newer cashier's checks are issued in a manner that makes them extremely hard to forge. Inks to prevent washout, patterns in void areas, etc...I mean its doable, but much more difficult than "adding an extra 0".
Yes I know this and you know this but proof has been given many times over that if it takes a thief a month to forge a dozen cashiers checks of various amounts than it is worth it to them. Then all they do is find items at or near that amount they can haggle to the amount the check is forged to and some unsuspecting person takes the hit.
Again cash and carry or go to the bank with that person while the check is drawn with you watching the entire transaction.