There was a discussion on Guitar Notes about writing songs without rhymes, John Lennon has a few, like Julia, which only has Shimmering, Glimmering within a line. The western songs like that which don't rhyme tend to have a lot of repetition of a central phrase or word, sort of rhyming with itself. And after thinking about it, it seemed to me many use dom 7's a lot for a swing-back effect, sort of like a rhyme.