On a duty/carry gun, the strain screw must be bottomed out on the head to prevent it from unscrewing during use. If the head is not solidly seated, they will back out from the flexing of the spring. Belt and suspenders insurance would be to bottom the head and use blue Loctite on the screw.

For range only, target and competition guns, I use the setscrew and blue loctite. Once the blue sets up, you can still make minor adjustments with an allen wrench and it will stay where you put it. This gives an easy way to find the lightest reliable trigger pull for any given ammo. If the screw should somehow back out, all you lose is points in a match, not your life. I have never had a problem, but not taking that chance on a carry gun. A half a turn on the strain screw is a huge amount. I usually adjust mine in 1/8 turn increments if I don't have a scale handy to see where I'm at.