There have been multiple studies conducted on this issue, the most recent being conducted by the Deutsche-Baeg Automotive Research Institute in Kassel Germany. While the study focused primarily on European automobiles, I believe it's findings are relative to this discussion.
The researchers studied 38 different cars over a 1-year period. Each car was thoroughly tested for control prior to the study, with the same testing being performed at the studies conclusion. Areas tested included, but were not limited to: compression, gasket condition, WHP, WTQ, hood paint integrity, front fender paint integrity, hood support condition, hood shape, etc. The cars were driven three times a day for 1, 2, and 3 hours respectively. 19 of the cars had their hoods propped open for a period of 1 hour after the drive throughout the study. The remaining 19 cars were parked with their hoods down. All 38 cars were parked in a climate controlled warehouse. The drivers, position in the caravan, and parking placement in the garage were all constantly varied and randomly selected to avoid contaminating the results.
At the end of the 1 year term the cars were examined by several experts from leading universities and research institutes. The findings had some variance from expert to expert, but all of them agreed with the studies finding. The study concluded, rather convincingly, that drivers should always either leave the hood up on their car after driving, or keep it closed.
