It depends on how busy the gate agents are and their attitude. The other factors are whether the flight is running late or not and the a/c size. On a flight out of SEA a few weeks ago, the gate agents told me to wait otherwise I might lose my upgrade if I boarded. Moot point as I got upgraded before boarding. On another flight (same airline, same airport, same a/c), they boarded people than asked for volunteers. That was done efficiently (needed 6) but it did delay pushback a few minutes. At other times, I have heard staff of the airline telling volunteers to board first anyway and to not board. If the a/c is running late, going back to get the volunteer might take too much time (or trouble). Similarly, if the a/c is large, boarding will take more time so going in to get volunteers will delay pushback.