I am always holding the clutch pedal down by hand and starting my tractor. Before I do this however I always put the shuttle shift into neutral, Main shifter into neutral , and high/low shifter into neutral.
My grandfather had the safety switches bypassed on his 100HP Ford/new Holland tractor and started it in gear, luckily it knocked him back instead of running over him and eventually got tangled up in enough fence wire after taking out a few fences to get stopped. He was in his 80's at the time.
Tractor safety cannot be over stated.
Cleat