Poetic license properly should be the ability to get at the real truth of the matter without being overly troubled with factual precision. Facts are such slippery little things; there are so many of them that some must always be left out, and by leaving out different ones you can arrive at entirely different "truths." If we must chose (and with our imperfect knowledge I'm afraid we often must) between facts without truth and truth without facts, I would definitely vote for the latter.
But poetic license too often is taken the right to stick in any word so long as it rhymes, even if it makes no sense grammatically or logically. I was reminded by this when I was trying to pick up some Fourth of July books for the ducklings and came across an illustrated version of "You're a Grand Old Flag." This set me to thinking about the lyrics. OK, "Every heart beats true under red, white and blue"--that's poetic extravagance, but it's a nice sentiment. But "where there's never a boast or a brag?" Bragging is practically an American virtue. It's like putting in a line about how gleefully Americans submit to orders. Just not the right touch at all. The whole problem was putting the word "flag" at the end of the line; there's just not many promising rhymes: Nag? Swag? Rag? Bag? Gag? Francis Scott Key had more sense ending with "wave": we may or may not be brave, but at least we'd like to be brave.
(Oh, wait! I have it! "Every heart beats true under red, white and blue/and our spirits never will sag." Hmmm.)
Do I over think this? Why yes, I do. It's murder to be in my mind during the song service at church. Once I'm done picking apart the lyrics and proofreading the Powerpoint, I can always critique the music, which on modern songs often seems to have been written up to a certain point, whereupon the writer got hopelessly stuck and just launched into a new tune.
Sometimes I wish I could turn off the internal critic and just feel things, but feeling things is so uncomfortable, so perhaps it's just as well.