Hi MoT
My copy of the 737NG Flight Crew Training Manual has the following note on p3.4:
Allowing the engines to stabilize provides uniform engine acceleration to takeoff thrust and minimizes directional control problems. This is particularly important if crosswinds exist or the runway surface is slippery. The exact initial setting is not as important as setting symmetrical thrust. If thrust is to be set manually, smoothly advance thrust levers toward takeoff thrust.
Note: Allowing the engines to stabilize for more than approximately 2 seconds before advancing thrust levers to takeoff thrust may adversely affect takeoff distance.
I no longer have access to a 'Classic' FCTM, but I assume a similar statement will be included.
I hope that helps.
Regards,
PA28pilot