Being complete honest all the time is a virtue for normal people like me. The politicians are also demanded by the citizens to tell the truth all the time, but they always fail to tell the truth. I think it is necessary for the politicians and the governments to tell the truth to maintain a good relationship with the people, and telling the truth can also raise the government's confident level. While in some special occasions, it is impossible and unnecessary for the politicians to tell the truth.
In their daily lives, telling the truth is a good virtue for the politicians, especially when their personal lives are in trouble. In 1996, Bill Clinton was impeached for the Whitewater scandal, although he is not deprived of being a president, the supporting rate was largely influenced by his cheating. In an other case, Gorge W Bush claims that the Iraq government possesses large amount of mass destruction weapon without any solid evidences which in my opinion is another form of cheat, then wage war on Iraq by the name of "world police". It was not until the American army bogged down in Iraq did the Bush government realizes their mistake. The cheating of politicians not only has a bad influence in their supporting rate, it also brought some bad effects on its country.
 In most situations the politicians to tell the truth, in some special occasions, however, it is impossible for the political leaders keep the information secret, because the politician represents a country or a party. A country is more likely in danger if it has no secret in national defense. In the time of second word war, Dwight Eisenhower cheated Hitler to let him believe that Normanding landing was only a feint attack and the main attack would come at Calais, where the channel is narrowest. Given that if Eisenhower did not cheat Hitler, the landing is less likely to success. Even today, the American national defense department keeps many secrets. If a country's defense system is exposed to public, it is hard to guarantee the country's safety, that is why ,I think, the Pentagon always was twenty-four hour guarded . In the aspect of national defense and safety, it is almost impossible and unnecessary for the politician to tell the truth all the time.
Before a truth that may cause chaos was to be told to the public, the results should be measured. There once a novel wrote that a scientist came up to a result that the end of the world was in three days, the politicians told the people the truth; then the world was in chaos, the store was robbed, no man went to work, the hospital did not save people, many men died form the unrest and lake of treatments. Given the information above, I think the politic leaders can tell a white lie to the citizens to prevent the chaos and turmoil when they are in a occasion like it is presented above. The real case can be also find in America, the NASA has many information about alien and the UFO we do not know. The United State government also keeps it secret to avoid the chaos in public.
We need the truth because we have the right to know them, we want the truth form the politicians and the government.  However, the politicians are representing a country, the national's interest is more important than the people's curiosity and the credibility of the politicians, so I think it is not necessary to know the truth all the time.