This has never been a requirement in the past. Until President Nixon’s famed “Checkers” speech in 1952, where in an attempt to clear his name over controversy surrounding his finances he decided to release his taxes. He voluntarily released his taxes through no requirement under the law. This practice is optional. Some president's release all of their info like Mitt Romney, while others only release partial like the Clintons did in the past. To date, nothing has been learned by this practice, so it's questionable what the value of doing this is.