FCTM: The Captain must keep his hand on the thrust levers when the thrust levers are set to TOGA/FLX notch and until V1.
Thats all the Flight Crew Training Manual says.
Personally, I teach people that there is no need to 'guard' the thrust levers during the climb. I find it quite disconcerting to see a young hand flash across the flight deck as soon as the wheels are off the ground, only to do ..... nothing!
As someone has already said, performance is calculated at Flex power alone, in fact this gives more than required as the actual temperature is lower than the assumed one.
I am a great believer that a considered action is more likely if the thrust levers are not being held at the moment of crisis after all, it takes but a second or 2 to push them to TOGA if it hits the fan, as it were.
After the above quote the next entry in our OMB (regarding TL's) is
AT THRUST REDUCTION ALTITUDE (LVR CLB FLASHING ON FMA)
THRUST LEVERS...................................................... .......CL
ANNOUNCE FMA......................................................... “THRUST CLIMB”
Move the thrust levers promptly to the CL detent, when the flashing LVR CLB prompt appears on the FMA. A/THR is now active.