Folk song (proper) is full of historical inaccuracies - sometimes deliberately so - they are more often records of people's desires and impressions rather than documentary information
The bard who composed Haughs of Cromdale actually invented a second battle in which the Jacobites routed the enemy, a pretty good example of this

For me, it depends on two things - how important the inaccuracy is and how difficult it is to alter - if what you have written captures the general spirit of the subject, why bother ?
Jim Carroll