As others have mentioned, it's context, context, context. Departures from the pattern -- or substitutions, if you prefer -- are expressive only insofar as there is a pattern to depart from. Sometimes the "departure" happens right at the beginning, as in "Let me not to the marriage of true minds." The context can be pretty big, a whole collection of sonnets or even the whole tradition of sonnet writing; still, the poem itself had better settle down to a more or less regular meter or there's nothing gained. How much "less" can a poet get away with? Try it and listen. I appreciate a poet who, like a musician, likes to show off a little bit, who shows that she or he can hit the right accents and then be a bit extravagant when the sense is better served by it.
RPW
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