Power arduino from 5v rail of pc molex I'm building a small device using arduino that should be connected to my pc via a usb header to exchange data with the PC.

What I'd like to achieve is that arduino turns on when the pc is on, whereas it shuts down when the pc is turned off. If I rely on the usb port to provide power to the arduino that might not happen, since some motherboards still provide power to the usb connections even when the pc is off. 

Therefore I'd like to power the arduino using the 5v rail directly from a Molex. I know that theoretically I can connect that rail straight to the 5v pin, but what about power spikes? How can I be sure that the 5v input voltage is properly filtered and stabilized? Would it be better to use a LM317 and transform the 12v rail into 7.5v to feed arduino from the vin?
The 5v of an ATX power supply will be perfectly adequate for powering an Arduino.

It will already be very heavily filtered and stabilised.