I think most of us generally stick to singing a song the way we like it best, but sometimes it can be beneficial to tailor the song for a particular audience as weelittledrummer describes. For example, in the Kildonan Hotel, where the audience were mostly tourists rather than dyed-in the wool folkies, I heard a singer announce that he wouldn't sing "Loch Lomond" in the traditional way, and sang it in a way similar to how Runrig do it. It went down very well. "Raglan Road" is another one which is sometimes is done agonisingly slowly, but would perhaps benefit from being sung faster and more rhythmically for the general public.