Not only "in the heads" when you've been reading this kind of statements in a so called Professional Pilots forum:
Posted by 8Che :
Every single ATC instruction whatever it may be is OPTIONAL for the Captain who is the only person legally in charge of an aircraft.
Let me extend this to runway stop bars. They are activated by the same ATC guy at his own discretion. Would you also say stopping here is optional ?
In complex airspace or airports, safety is based on certain regulations and rules that must be followed . Period. Safety is not only for you and your aircraft, it is definitively that of the others around you too.