There are guides to standard practice -rules, if you like- but you might have to ask a specialist where they can be had. In the case of folksong, many of the early collectors were not experienced, and their approaches often differed; when I was working on the "Penguin" revision, for example, a number of inconsistencies came up. The music setter was able to advise on the "right" way, and in most cases we went with that; though in a few it was obvious that the original collector's notation, though non-standard, more accurately represented the way the singer had voiced it, and in those cases we ignored the rules (as also in a few cases where doing it "properly" would have resulted in confusing notation with certain sounds apparently assigned to the wrong notes).

The standard approach doesn't, I've found, always reflect the way words actually need to be pronounced or divided; the criteria seem to be mixed between the visual and the aural. A compromise for the sake of manageability, perhaps; but if you do find an accessible reference for the standard "rules", do let us know. It would be quite useful.