There is a lot of variation across models - some supplies will latch and hold without a preload; some will not. Later ATX versions will generally hold while the earlier production models will require a preload on the 5V rail -- if your PS does not power up when shorting PS_ON (green) to ground, then a load resistor may be needed.

Design Guide updates of importance to individuals converting their power supplies. The +3.3 VDC remote sensing wire (brown) is connected directly to the orange +3.3 VDC at pin 11 of the power connector (pin 13 if a 2x12). The sense wire will usually be of a lighter gauge (22 AWG) than the power wires (16 or 18 AWG) and its purpose is to monitor the voltage at the connector in order to provide feedback for voltage compensation by the supply.

If your PS has two wires attached to the same pin on the motherboard power connector, orange +3.3 VDC and a brown sense wire on Pin 11 for example, then these two wires should be joined when you make your conversion. Some power supplies may also have sense wires running to the +5 VDC and +12 VDC connector pins. If there are multiple sense wires, then they will usually be of the same color as the primary supply wire, but of a lighter wire gauge. These sense wires will also terminate in a different location on the PS printed circuit board than the heavier supply lines. As with the +3.3 sense wire, these additional wires should also be connected to the corresponding supply lines. Failure to complete these connections may result in your power supply not latching into a Power_On mode when switched on.

A cautionary note: Most supplies will have at least two smoothing capacitors located downstream of the rectifier -- these are relatively hefty caps and may give a good jolt if touched immediately after unplugging the PSU. Prudence would dictate either grounding these or initiating power_on without AC before mucking around much with the board.