Insert something quick here while we wait for Mr. Johns to reply; unprepared modulations sometimes are the most interesting ones. If you doggedly prepare each modulation with the dominant of the new key, it telegraphs your intent, taking away any surprise of freshness that might otherwise be gained.
This happens all the time in classical (pervasive in late romantic and impressionist literature), jazz, and pop music. A couple of clear examples to quickly cite:
(1) Hey There ( jazz standard), the first A section is (typically) in Eb major, the second modified A is in G major (a chromatic third relation between these two keys). If you prepare G major with a D7, its OK and many times done that way, however its fresher if you do not prepare G, you get the sudden appearance of F#s and B naturals, that were MIA in the old key.
(2) Julia (the Beatles), The verse is in E major, the first chord in the bridge is an abrupt Eb minor (tonally a long way away). The absence of a dominant preparation to that chord makes the change all the more dramatic.
Thats the funny thing about theory; you need to know all of that content, but worrying about all the right ways and wrong ways can stifle your creativity. I say learn from the masters.
JS
