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On many vehicles the wipers cause an insignificant amount of drag. IMHO the designs that approach "1 box" i.e. the hood and windshield are nearly the same angles probably have more sensitivity to wiper design and placement, but the others with the more abrupt transition angle tween hood and windshield have bubbles of relatively still air at the base of the windshield where the wipers live. Hence, the wipers aren't sticking out into a mass of moving air like everybody thinks. Two examples: in the rain, notice the drop patterns on the glass. On my cars drops just sit there still in the bottom 1/4-1/3 of the windshield while on the upper part they blow away. Another example: one time I checked air in the tires and forgot the valve stem cap on the cowl. 250 miles later, I noticed it still sitting there! It would have blown away anywhere else on the car.
There is a Hot Rod Magazine article that mentions windshield angle. They say flattening it, in and of itself, doesn't help aero. I suppose it could if it allowed better flow at the top of the windshield/roof interface.
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