Does an ATX PSU's PS_ON Signal Require Resistance
There is no need for a resistor. Any voltage &lt;0.8 (or ground connection) will enable the psu

Refer to ATX12V Power Supply Design Guide section 3.3.2


  PS_ON# is an active-low, TTL-compatible signal that allows a motherboard to remotely 
  control the power supply in conjunction with features such as soft on/off, Wake on LAN*, 
  or wake-on-modem.  When PS_ON# is pulled to TTL low, the power supply should turn 
  on the four main DC output rails: +12VDC, +5VDC, +3.3VDC and -12VDC.  When 
  PS_ON# is pulled to TTL high or open-circuited, the DC output rails should not deliver 
  current and should be held at zero potential with respect to ground.  PS_ON# has no effect 
  on the +5VSB output, which is always enabled whenever the AC power is present.  Table 
  14 lists PS_ON# signal characteristics. 
  The power supply shall provide an internal pull-up to TTL high.  The power supply shall 
  also provide de-bounce circuitry on PS_ON# to prevent it from oscillating on/off at startup 
  when activated by a mechanical switch.  The DC output enable circuitry must be SELV-
  compliant. 
  The power supply shall not latch into a shutdown state when PS_ON# is driven active by 
  pulses between 10ms to 100ms during the decay of the power rails.