I think whether or not agents will have volunteers wait at the gate is solely based on the likelihood of needing volunteers and is a judgement call. I usually ask about volunteering on return flights and even when the flight is oversold up to 10 over, sometimes the answer is still no. I've also boarded fights that sold as full, and ended up volunteering after being seated. A volunteer told they are not needed faces full overheads...this is the downside to volunteering. I think volunteers that board should be given some compensation, maybe even something as little as a drink/headset voucher.