I get out at sunrise when it's completely calm and zero the 300 yard BDC mark at 300. Then I shoot the 100/200/400/500 yard mark to see where they are hitting with my load and compare it to the data from my balistic software and make notes.
If the Match only has 50, 100, or even 200 yard long range shots I'll zero to those distances to eliminate the need for hold over/unders.
The reason I like to zero at 300 is I've had a nice 1-2" group at 100 or 2-3" group at 200 start to move 3-5" off center as I stretched it out to 500-600. Heck if I had the distance I'd prolly zero at 400 or 500 because if you're dead nuts at 400-600 the closer stuff is going to be deader nuts!