The IPSC Rifle (and I believe shotgun) rules up until 2012 specified that the gun should be parallel to the ground, stock touching at hip level. The parallel to the ground rule was then removed and now most sensible people start like this:
It makes it easier to get first shot on target since the end of the barrel is already in line with the target, so you only extend your left arm bit and bring the stock up just like in shotgun sports (at least outside US) I used to shoot sporting clays and international skeet before IPSC rifle, mounting the rifle is just the same in IPSC as in those sports.
Also, in IPSC, down range is anywhere within the safety angles, and up range is the opposite. You will always start pointing towards your first target to save time.