Roughly the 12 and 6 position, especially when standing. Lag in torque is not a significant factor.

This is an easy experiment to do. If you shift at the 3 and 9 o'clock, the shift can actually happen faster, but it will sound and feel very different. Despite what people say, it's impossible for torque to be the same all the way around the circle -- that's just not how your legs work. Anyone who doubts that should see how fast they are off the line starting from the 12 (or better yet, 10:30) o'clock position.

I have found that shifting under load is a great way to spread some FD's which leads to mucked up shifting and thrown chains until you fix it. I have this issue with my FSA FD, but not with my Ultegra or 105 FD's.