I think it varies on track size, shape and how hard/easy the first corner is going to be.
Oval racing on a short OVAL - we start single file on the back straight for heats, but double file for mains.
Road racing for club races we do IFMAR/STAGGERED clock starts for heat, but double file "Shotgun" start for the mains. But we have a large track, and the racers are good enough to blend going into the first LH 180 degree corner.
For VT/A - we line up diagonally in single file on the inside wall, and we don't use IFMAR style clock, and that's kind of a kick in the butt.
R/C Racing since 1985 ~ Santa Maria, CA (Central Coast)
2001-2012 - South-West Tour R/C Oval Series - R.I.P.
...will the SWT be revived? Time will tell, but we ARE returning to the VELODROME in 2016