Depends on everything

But you need to learn how your car accelerate and when it's loosing traction, a good combo of both

Its always faster not to spin the wheels, but also harder for the transmission.
Also flat shifting from 1-2 is were you can catch some time on alot of stock cars xD (Hold the speeder, quick push the clutch and fast pull the gearlever, without letting go of the speeder)
Some cars will "jump" when shifting too fast
and if you car pulls from 2000-4000 (TDI for example) you don't have to rev it over 4000rpm before shifting gear (you will lose power after 4000 and its better to shift gear)
ALWAYS traction off (unless it's a supercar build for racing) Because if you start, and the car start to spin it will reduce power (Much faster/smoother to let go of the speeder and re engage)
On an Auto Transmission,find a good rev to launch at, put it in D, hold the brake, rev it (to the sweet spot) and then just floor it...