You should use steptronic whenever you know that you are going to be accellerating and decellerating rapidly. For example, if you're going 75 on the highway and you want to pass another car, you can tell the transmission to downshift to a lower gear. If you simply floor the pedal, it will take the car a moment or two to realize that you need more speed quickly and the downshift will be delayed. I also like to use the manual mode when I'm driving along very twisty roads, and I want to power out of a turn. When a curve is approaching, I'll brake and downshift at the same time, that way I get alot of power coming out of the curve. If you leave it in automatic mode, the downshift will again be delayed beacause the car doesn't know what you're going to do next, only you do
Hope this helps.