It is more of a safety issue to hold the baldes at the centre(yoke or bladeholder) so that if the engine starts and the throttle is open too much or the idle-up switch is on the clutch would engage and start to spin the blades and if you don't have control of the heli you or someone else may get injured.Where as if you hold the blades with one hand and start the motor using the starter(assuming it is a top-start or belt started) with the other hand no one can get hurt.The only thing that will happen if the engine is idling to high and you're holding the blades the clutch will burn out after a while.
Also make sure you have your radio next to you and are in the sub-trim menu or use the mechanical trim lever (if you don't have a computer radio) to lower the idle if you start the engine and it wants to turn the rotors.