when doing regular squats, the bar usually sits in a groove from my lower traps and rear delts. by pulling down and foward on the bar across your back, the bar is anchored.
That pretty much describes where I hold it as well. Its personal preference actually and some genetics on how your muscle lay. A friend of mine always carried the bar real high, right at the base of his neck on top of his traps. That just doesnt work for me so I carry it lower in the groove jesus 2 described.