Separately, what is the practice amongst story tellers ? That seems very close to narrative ballads.
The traditional storytellers were the repository of history therefore the emphasis was on accuracy of transmittal, presumably without a third party exerting a "malign" influence. That is if you tend towards the idea that story tellers are the modern descendants of the bard and the filidh(in Irish terms)

To suggest the Mabinogion or Táin Bó Cúailnge (The Cattle Raid of Cooley) were subject to the folk process, would I suspect, be regarded as rank heresy. But others may argue otherwise.