Like many other speakers, I'm quite comfortable using quotations that are pithy and relevant, even if I can't document their sources. In academic mode, however, I know I need a footnote. For a long time, I quoted Mark Twain as saying, "History doesn't repeat itself, but sometimes it rhymes." I've now seen enough commentary that I feel constrained to say, "As attributed to Mark Twain." It seems that almost all really good quotes come from --or have been attributed to -- Twain, Churchill, or Yogi Berra.
The same with stories. I've often cited Lyndon Johnson's comment supposedly made to a sergeant at a military base:. The enlisted man had said, "Not this one, Mr. President, your helicopter is over there." And Johnson replied, "Son, they're all my helicopters." [Sorry, this isn't in the Public Papers of the President.]
The discussion about the White House report on war powers in Libya -- where the White House lawyers defended the President's actions -- reminded me of another LBJ story too good to verify. It's said that he called a newly named White House counsel into the Oval Office and raised an important question" "What I want to know is, are you a "yes" lawyer or a "no" lawyer?" [He should have known that almost all government lawyers, perhaps except in the White House house, are congenitally "no" lawyers, just as most lawyers in private practice as "yes" lawyers.]