The wiper on the R18 barely functions - the tip of the blade is not in contact with the glass in the parked position, and they have all they can do to get it to wipe the water off of the extremely curved windshield. Given those factors, the best place to park it is straight up in the center.
But for a regular car, with a much broader and flatter windshield that requires two wipers, parking it any way other than horizontal is worse. Interfering with the flow around the 'A' pillar is a bad thing. The i MiEV is an example of how not to do it:
The Smart ForTwo has a similar proportion windshield, and it is a much better solution:
Tuft testing will show that most wipers in the parked position are in still air. I have had maple seeds and such sit on the wiper and they move slightly only when there is a side wind gust. Otherwise they just sit there. Light rain mist hits my xA windshield almost 1/3rd of the way up - and that is with a very short and steeply sloped hood.
