I always adjust drag brake at the transmitter using the throttle trim. There are two advantages to doing this:
1) I can change the setting instantly from the transmitter.
2) Setting drag brake on the ESC means the brake will instantly engage at the drag brake setting when I lift off the throttle, which can unsettle the car on corner entry. The action is much smoother when the drag brake is set with the throttle trim, since there is a gradual transition from power to coasting to brake engagement as I lift off the throttle. Imagine the reaction you would get from a real race driver if you told him that as soon as he touched the brakes, they would engage at 30% (for instance). He'd probably refuse to get in the car!
One drawback to setting drag brake with the transmitter throttle trim is that many ESCs won't "boot" unless the throttle is at neutral. My work-around is to set the receiver fail-safe position to neutral, then power up the car before the transmitter.