Personally, I'd be as happy with 'To-ge-ther' as 'To-geth-er', especially if the second syllable was spread over half-a-dozen notes, and I'd prefer 'bro-ther' to 'broth-er' because I could imagine myself singing 'broth' and then realising I had the wrong vowel sound. That's why I don't think it would be easy to come up with reliable rules - English just isn't that obliging. The only rules I think are really worthwhile are:

i) Never steer the sound in the wrong direction
ii) Make the syllables as recognisable as possible. I think this leads you to follow the dictionary style except where it conflicts with rule (i).
iii) Once you've sorted out (i) and (ii), the syllables should fit back together to make the word. Without this, you risk confusing 'lose' and 'loose' for example.