Depends on what you mean by necessary. There were more than a few U.S. Generals who didn't think so. Adm. William Leahy (President's Chief of Staff, Adm. Chester Nimitz, Adm. William Halsey, Gen. Douglas MacArthur, even Gen. George Marshall. Their accounts can be found here

Historian Gar Alperovitz (Atomic Diplomacy: Hiroshima and Potsdam) has said that Japan would have surrendered well before a November 1 deadline, especially due to the fact that Imperial High Command was more concerned with the Soviet Invasion of Manchuria than they were with the Atomic attacks due to overshadowing in Japanese tele-traffic speaking more about the Invasion.

Another note is that at least a dozen Japanese cities with populations of more than 50,000 had been firebombed and virtually flattened (most portions anyway) during the early summer of 1945. News of Hiroshima didn't reach Tokyo until August 10 (4 days after), with verbal reporting to the military by August 8 (2 days after). On August 8, Foreign Minister Togo Shigenori went to Premier Suzuki Kantaro and asked that the Supreme Council be convened to discuss the bombing of Hiroshima, but its members declined. That should put into context the that the Atomic bomb dropping was priority.

Japan was clearly losing the war. It is my belief that the firebombing by the Air Corps and the subsequent Soviet Invasion would have pushed them into unconditional surrender by the end of August. However, that is just my thought.

But you said necessary to use it, and to an extent, yes. It was. The world has not seen warfare on the scale of WW2 since 1945, mostly due to the development of the atomic bomb and the subsequent M.A.D. Doctrine that followed. 70,000,000 people died in World War 2. Around 5mil in the Korean War, and 4.2mil in the Vietnam War, and 2mil in the Soviet-Afghan war.

If you put all the wars together from 1946-2016 (70 years), you would just barely overtake that of war from 1939-1945. That is mostly due to the development of Atomic weaponry and moving away from conventional warfare as a result and the M.A.D. Doctrine that followed after the Soviets developed their own weapon in 1949.

edit - grammar