Mike,
The idea (and some requirements) for stress relieving after grinding have been around for a long time. It will certainly hurt nothing to stress relieve your parts. My suggestion for elimination in your case stems from work I have done on 4340 alloy steel which showed no effective stress relief will occur below 600F unless the time of exposure is very long, probably hundreds of hours. The temperature threshhold for stress relief in your PH stainless is likely to be much higher than for alloy steel.
