It takes serious dedication to time your visit to when the customer-friendly employee is working in order to maximize your french fry intake. Serious dedication.
McDonald's, not a Reddit username but the actual business, denies the allegation. Terri Hickey, a spokesperson, told the Huffington Post, "The notion of a secret trick is absolutely false. There are strict procedures in place to ensure that fry boxes and bags are appropriately filled so our customers can enjoy our World Famous Fries to the fullest."
It's possible these are anecdotes taken from franchisees gone rogue, or maybe the allegations are truly false. Either way, you'll know what to keep your eyes out for if you're worried your bag is missing three or four fries. Also, consider finding out who the Psychoopiates is at your local drive thru. They're on your side.