What is the correct way to interface 3.3v logic with the PS_ON# 5v signal on a standard PC power Supply?
Use a NPN transistor. PS_ON to a 1k resistor at the collector, 1k resistor at the base, emitter to ground. Voltage at the base can be different than the collector.

Update: Further research shows the PS_ON tends to have a pull-up resistor to 5vsb. In that case, adding a 1k to the collector creates a resistor divider. If too high a ratio, then the input signal might not cross to a Input Voltage Low. You can size down the resistor, or omit it.