You don't need a check valve, not sure where that idea came from. Just make sure you clean the stone when you clean your keg.
Have stones on all my serving kegs and they work great. I use the .5 micron stone that AHS sells. Stone is inserted into a piece of tubing, the other end of the tube goes onto the gas in dip tube. Use a 2' length to reach the bottom of the keg.
"Why did you.... what was the point of... how drunk were you when you decided this was a good idea?" - DMartin