I have powered my UNO using the 5V pins from and atx desktop SMPS. I have used the PURPLE wire from the 24 pin connector.
Though 5 rated at 5V without load it is 5.25V and with the arduino powered
(UNO + Ethernet Shield + 2Relays)
The Voltage measures to be 5.22V under load…
1#The UNO is currently being fed from the 5V pin on it… Is this a good idea or should i just feed it from the JAPAN JACK socket
2#Is the 5.22V alright… It is a known issue because certain SMPS do not regulte supply unles under a minimum load.
The purple wire should be okay. Just try putting some load on the SMPS.
Don't use the RED wire, if something goes wrong accidentally, it can supply 25A current and fry everything.
BTW, What's your location ? India?
1#The UNO is currently being fed from the 5V pin on it... Is this a good idea or should I just feed it from the JAPAN JACK socket
If by "Japan Jack", you mean the "barrel" connector, you can't. This feeds the on-board regulator and requires at least 7 V to regulate down to 5 V.
2#Is the 5.22V alright... It is a known issue because certain SMPS do not regulate supply unless under a minimum load.
The purple wire is the "pilot" supply in the ATX system, it is a completely separate (usually) second supply with a much lower capacity and intended to operate at little or no load - indeed, this is a requirement of the more recent energy certification if I correctly recall, that it draw less than one Watt, corresponding to no more than 200 mA, if that. So it should be fine to power the Arduino. 5.22 V is perfectly fine, well within the (5.5 V) rating of the Arduino.
(Some "mini" computer designs, and all using a separate "brick" power unit, contain only one power converter, presumably switching the primary supply using FETs, and again must therefore be designed to operate reliably under no load.)