You're right, I've seen it done several different ways.
At the hospital I currently work at we do the following:
Start at the incision line (usually lower transverse, right above pubic hair line) then go back and forth up to bellybutton (while not taking prep off of skin)
Then down into pubic hair area, go back and forth, then down both legs, logic being this is the dirtiest area).
If its to be a classical incision, we go down the middle of the belly (but don't touch pubic area) then go around and around (again not lifting prep off of skin) finally do pubic hair area.
But if you talk to 10 different people, you'll probably get 10 different answers. You're institution should have a policy on this, or ask someone from surgery.