Really, if you want to know, spend some time at the drag strip.
I found with the non-GTR or formula cars, you want to make your shift before the shift light comes on in 1st, 2nd and sometimes 3rd gears. This improves acceleration off the line. Maybe its just my "human lag" and even though I'm shifting earlier, I'm really shifting closer to the optimum point than if I waited for the shift light, but it is quicker.