Okay, so you’re writing a poem in a form that requires a fixed number of syllables or a fixed rhythm. You find a way to do it that will require contracting a word to reduce its syllables or change it’s rhythm. Maybe it’s “Towers” down to “Tow’rs” or “Murderers’” down to “Murd’rs’.” Try again. “Don’t” is a contraction you can use in a poem, but getting fancy will distract the reader. The reader is so busy trying to figure out how the contraction is pronounced or what it’s a contraction of that he forgets what he is reading about. Stick with the tried and true contractions, even the poetic ones, but don’t go inventing new ones. So, you then have to rewrite that whole line to conform to the rhythm or syllable count? It’s okay. You’ll come up with something better.