Wow, I tell you what, this chicken forum has more dog trainers on it than any other forum I visit.
You know the only thing two dog trainers can agree on?
What the third one is doing wrong.
Herding instinct bred into herding dogs is only a modification of the prey drive instinct. Many herding breeds also use 'gripping', which is essentially biting and is an accepted practice in some venues.
Heel nipping can easily turn into biting. I have seen this happen in border collies...