Does ATX specify the load currents?
The power or current ratings on ATX PSUs are maximum values. They show the max. available power at 12V, 5V, 3.3V and so on. Of course, lower loads and currents are allowed. Also your computer does not always use the full specified power. However, a small load on the output of switching power supply is always helpful for more exact regulation. A few mA should be sufficient. If you do not have very light loads or don't need precise regulation, there are no further steps necessary. 

Advice: Check the ATX plug specification to turn an ATX PSU on. In general you have to connect the green wire of the 20/24 pin ATX plug to Ground.