MaJoC's €0.02: I remember reading somewhere that songs sung in English tend to have one note per syllable, and one syllable per word, and that that's how you tell when a song was composed in English.* What I do myself in practice depends on the song and the circumstances: in The Handweaver and the Factory Maid, the rhythm comes from the words, while in shanties forebitters, the words have been wrapped round the rhythm. I'll leave it at that.

* Origin unknown. Enlightenment humbly requested.