It depends upon many factors, some compaq servers with redundant power supplies are not redundant unless you give them high voltage. This is because you get more wattage out of the high voltage settings. This is typically the case with anything with 4 processors and can also be the case if it is a server (or disk array) with a large number of drives.
Also High Voltage is generaly cleaner and more efficent (the meter does not spin as fast)
Bottom line, If you have access to High Voltage and the device supports it, use it.