[Christ]

It's a timing thing. Regardless of what order you do things in, it's not a good idea to rev your engine immediately after starting it, especially when it's cold.

Most of the people I know, don't slip-start their cars, when driving, therefore, they're going to rev the engine up a bit to take off.

Since it takes about 15 seconds to release the e-brake, put the car in gear, and put my seatbelt (4 point harness) on, I use those things to stop me from attempting to take off without giving the engine a chance to smooth out.

I can understand the need to save fuel by performing those actions before starting, but regardless of when you do them, chances are, you're still going to sit with the engine on for a few seconds anyway.