Fine Arts Model Question:

Only when doing a teaching workshop. Never on professional commissions. That is not the place to be teaching (though spectators are always welcome). Professional mural painting is not a democratic process or a paint-by-numbers enterprise.

Non-professional help dilutes the quality of the work. Let's go back to the brain surgery analogy: would you like some volunteers working on your head? I do however, take experienced art students as apprentices.