My personal opinion is that better cooling is produced when the air blows directly across the drive. However, as long as the drive temperature is kept within the environmental specification limits, it does not matter. The product manual for each specific model number or family of drives includes exact specifications for the direction and volume of air required by the drive, as well as all other environmental specifications. The operating temperature range for most SCSI drives is 5 to 50 degrees C. A normal PC case should provide adequate cooling, however if your enclosure is unable to maintain this range, we suggest that you contact your system manufacturer for information on cooling and ventilation hardware that is compatible with your specific configuration.
Here is an example graphic pulled from the product manual for the Cheetah 36 product manual: