From Transport Canada
RAC - 1.0 GENERAL
"Do not select “STANDBY” while changing codes as this will cause the target to be lost on the ATC radar screen.
Pilots should adjust transponders to “STANDBY” while taxiing for takeoff, to “ON” (or “NORMAL”) as late as practicable before takeoff, and to “STANDBY” or “OFF” as soon as practicable after landing. In practice, transponders should be turned on only upon entering the active runway for departure and turned off as soon as the aircraft exits the runway after landing."