It varies from one POCO to another. Some like to use one high voltage line and neutral/ground, others use a pair of high voltage lines, 2 of the 3 phases.
Also there is variation of how the secondary 120/240V lines are strung along poles. When they use 3 separate wires. Some places will have the neutral as the top of the 3 wires, other places will have the neutral as the middle wire. An arguement for the top wire being the neutral would be that it's the wire that would get touched by falling high voltage distribution wires. Thus shorting directly to ground and less high voltage getting into customers' homes.
