All I can say is your 6.0 will start a whole lot better if you wait for the light to go out. Not only do the glow plugs heat up, you give the fuel pump a chance to get the fuel pressure up in the system. It also allows the electronics a chance to get ready to run...
So I guess if you know more than the engineers that designed and built your truck, and for some strange reason don't have the 6 or so seconds to wait, do it how you want...
But I say there are enough
potential problems with these trucks to start doing things like this