I have straight wired both PS-ON and P.G. to the I/O on an UNO. Both are TTL logic per ATX standards and do not need limiting resistors or pullups. I would caution against sending 5v out to PS-ON. Don't set output high, just LOW. Code as digitalWrite(pin, LOW) and toggle between pinMode(pin, INPUT) for off and pinMode(pin, OUTPUT) for on (open and ground).
Also, watch out when using standby power to 5V input rather than 12 volts to Vin. using the USB at the same time can cause problems. If you use Standby power directly to the 5v input you bypass all the power protection and switching logic, make sure you turn off power supply or unplug before connecting USB and disconnect USB before turning on ATX master switch(and standby power), otherwise your ATX could send power to your computer. This means that serial communication via USB will not be possible when running using standby power.