Does a switching supply always need load
Generally, switching power supplies have a minimum percentage load to keep their output(s) in regulation. For example: a wall-wart type power supply rated for 12V may have a 16V output when measured with a DMM and no load. At very low loads, the power supply may also exceed its rated output ripple.

This problem, particularly for re-using computer power supplies, is generally solved by placing a power resistor that will draw the required minimum current across the affected supply rail. However, this decreases the system efficiency.

Additionally, depending on the configuration of a multi-output power supply's internal rails, the minimum power requirement may apply per-rail or to only a specific rail. For example, if the PC power supply generates 5V from the 12V isolated converter, externally loading the 5V rail would stabilize both the 5V and 12V rails. Check your datasheet, if available, or engage in trial-and-error testing for your specific supply.