3 rules of musical prosody

 1. Strong syllables fall on beats 1 or 3.  [Generally, linguistic and musical beats are aligned.] 

2. No enjambment! [Generally, linguistic and musical phrases are aligned.]

3. Melodic contours (shapes, up and down movements) line up with the phrasing of the line.  

The rules can be broken, but... breaking the rules produces tension or awkwardness. Not breaking the rule is (almost) never noticeable. I don't take it as breaking the rule if the strong syllable anticipates the 1 of the measure by an 8th note or less. But the listener might feel some tension.  That tension is built into the genre, in the case of jazz, where you are supposed to anticipate or delay a bit.  

The music of the music is stronger, takes precedence, over the music of the poetry, which can be more subtle and varied. It can seem to destroy the text, then. But the revenge of the poem occurs when the listener gets irritated if the poem is stretched too far out of whack.  You can override my rhythm, says the poem, but you will pay for it.  

When I set some Niedecker poems, I followed the rhythms and contours of the poem as much as I could, but I noticed a flaw, that I set the word "cranberry" as CRAN-be-REE instead of CRAN-BE-rry.