And it's whispered that soon
If we all call the tune
Then the piper will lead us to reason
And a new day will dawn
For those who stand long
Here "soon" and "tune" obviously rhyme, and "long" is pronounced as "lawn" to rhyme with "dawn." The use of rhyme here mirrors the kind of voice that Plant is putting on. It's more assertive and the poetry of using rhyme makes these lines seem absolute or truthful.
In Plant's final verse, following Jimmy Page's majestic guitar solo, everything is made to rhyme or have assonance. The ends of the first lines—"road" and "souls"—get their final consonants slurred to emphasize the assonant vowels, and the same happens to "gold" when it is rhymed with "show" and "know." You might think the next lines, "And if you listen very hard / The tune will come to you at last" can't possibly rhyme or have assonance, but Plant pronounces "last" with a weaker "a" as in the British pronunciation of "class" and "pass" in order to create assonance. And finally Plant wails out the ending consonance of "When all is one and one is all / To be a rock and not to roll." Live, Plant sometimes makes "all" and "roll" rhyme completely. Again, the increased use of rhyme and assonance swell with the song as it reaches this point of prophetic rock n' roll truth and emotion.
Another thing worth noting: "Stairway to Heaven" is full of Romantic figures of nature, enchanting beings, and big philosophical ideas like "reason" and "the west." These features, found in every verse, evoking the pastoral and mystical, are vague enough to be universally evocative and they're wrapped in a semblance of narrative (the narrative of the lady trying to buy a stairway to heaven) just enough to be engaging. There is never a clear picture of what exactly is going on, but Plant's lyrics bring together enough "metaphysical" material to appear to have deeper meaning while still resisting absolute interpretation. This parallels the band and cover art of Led Zeppelin IV; Led Zeppelin was notoriously camera-shy, for being the biggest band in the world, and IV's cover—an over-papered wall with picture of a hermit with sticks on his back—was utterly impenetrable in terms of meaning. This creates an occultist, mystical fascination for those who want it. Even if the words have no absolute meaning; they allow for endless debate and wonder.