I see where you are coming from but were I interviewing you I would never even think to wonder about a self reported grade in a volunteer class. If the topic was important to the position I'd ask you to talk about it and tell me what you learned. I would hope I could spot you trying to feed me a line.

At the end of the day, as an employer, I am looking for 'learners' not 'cheaters.' If it turns out that an employee's personality/choices lean toward the cheating side I try to manage them out of the organization as smoothly as I can.