There are numerous different version of this quotation. When he first said it, it presumably was in the present tense (and is often recorded as such). In a Wikipedia entry about the one-man play it notes "Whilst in Harrisburg, Illinois, Truman delivered a speech attacking the Republicans. During the speech a supporter yelled out "Give 'em Hell, Harry!". Truman replied, "I don't give them Hell. I just tell the truth about them and they think it's Hell." Subsequently, "Give 'em Hell, Harry!" became a lifetime slogan for Truman supporters."
However Time magazine, Monday, Nov. 13, 1950, reports his retort to "Give 'em hell, Harry!" as the not-so-catchy "All right, I'm doing it."
Canadian Prime Minister John G. Diefenbaker adapted the line: Someone shouts out: "Give 'em hell, John!" and Diefenbaker responds: "I never do that. I just tell the truth and it sounds like hell." Alternate versions have Mr. Diefenbaker setting up someone in the audience to shout out "Give 'em hell, John," to which he immediately responds "I don't do that. I just tell the truth and it sounds like hell to the Grits." ("Grits" was a nickname for the Liberal Party of Canada, the hated opposition.)