These are good questions.
First, there's a thread over on the grbl 'issues' list on github regarding this exact issue: https://github.com/grbl/grbl/issues/51
Although lengthy, It's worth the read from top to bottom.
As for an 'estop': On my rig I use a 6 plug adapter to supply power to the 24vDC power supply and the spindle. In a true e-stop situation, it's my thought that you want to kill the power/stop the machine as fast as possible. Period.
There is no positional feedback from the stepper, if you computer were to remain on after you powered down he steppers then no, it will not stop streaming the gcode to the arduino and your position will essentially be lost.
Also, there is some capacitance in the power supplies, so even after throwing the reset switch on the 6 plug adapter, the steppers will still move some distance before the power is completely gone. I've been meaning to put an actual estop switch between the DC power supply and the shield, which would essentially kill the power to the steppers 'right now', without a delay.
I go with the 'big red button' theory for shutting everything down and think that if all else fails I'd rather scrap a part than risk either injuring myself or breaking the machine or some of it's components.
Shapeoko #0 - a couple of upgrades.