I used to do my burnouts starting in 1st and immediately going to 2nd, then 2rd, 6000 RPM max, and after a few seconds in 3rd you could hear the motor being dragged down a bit; let off the line-loc button, lift smoothly, drive it out.
Now (new trans w/low-band apply) I start in 2nd, go to 3rd pretty quickly, otherwise same procedure.
I noticed the line-loc holds the car better starting in 2nd. Starting the burnout in 2nd is allegedly easier on the sprag, or so I've been told.