Hi Stan.
As the slots in the screw heads are cut with a slitting saw when manufactured in quantity the slot sides are parallel the same would also be true if the slot is cut using a hand piercing saw (jewellers saw). The V shape you are thinking of possibly comes from the way armours screw drivers are hollow ground to give a very close fit at the top of the screw head slot to prevent damage to the slot by the screw driver caming out when you turn it. There is also a way to make the screw driver dig its self into the walls of the slot this is accomplished by giving the screw driver a couple of sharp hammer taps on the blade which produces a small bur at the end.
