A recent episode of the television show "Bones" happened to include two rhyming word pairs: roach coach and tramp stamp. This phenomenon of rhyming word pairs is often considered a form of reduplication. Reduplication is the linguistic process of repeating all or part of a root word. Reduplicative or not, I believe there is more to this type of rhyming word pair than is apparent in the initial semantic interpretation. The more I am referring to is idiomaticity.
Take the rhyming word pair tramp stamp for example; each word in the pair can stand alone, and when the words are paired the meanings of the individual words are still included in the semantic interpretation of the pair; however, there is more to the meaning of the whole than the combination of the meanings of the parts.
tramp = woman of loose morals
stamp = a lasting imprint
tramp stamp = a tattoo on a woman's lower back
Additionally, this type of rhyming word pair should be considered idiomatic because if an aspect of either of the individual words is changed it will lose the meaning of the whole, in other words its idiomatic meaning.