100% agree that prime IBT is not a sole indicator of if a PC will fall over when clocking. It is useful with other programs to determine how effective your cooling is.
If you stay within the recommended power envelopes and you can run everyday tasks, games with impunity (ie no BSOD, restarts), it should not be necessary to run stress testing with a moderate OC (20%).
If however you are mission critical, mining deep space for radio signals 24/7, creating local brownouts with your power consumption then probably yes you should run a stress test