I thoroughly agree with georgeward too. Above all the words have to sound good when you sing them. I have never thought that it was about finding some words to fit a tune or vice versa. They have to wrok off each other. Usually the words and the tune take you in the same direction, but sometimes they are deliberately taken to contradict each other. One example of that is the last verse of Randy Newman's "I think it's going to rain today".
    Right before me signs of glory
    Help me live - show me the way
    Human kindness is overflowing
    And I think it's going to rain today.
When you put that into the context of a great song, the effect is heartbreaking. I think that's the difference between songwriting and poetry.