The answer is depend of process system. The system is very unique and may be different from one to another. When we will select fail-safe position, it is better to understand process and safety consideration. However guides below will help you to select fail position of control valve.
When to use fail close position?
Preventing overflowing a tank
Control valve discharging wastewater should be fail closed to prevent release of toxic material contained in wastewater
When to use fail open position?
Control valve controlling cooling water flow rate to condenser should be fail open so that remaining hot fluid could be cooled by the cold water thus preventing overheating and damaging critical component
To prevent pressure build up in tank due to pressure relief valve failure, we use control valve in fail open position.
Control valve installed at minimum flow line should be fail open position