I have shot my 642 from a retention position several times, including +P ammo. The biggest negative I saw was more unburned powder than usual on my shirts. However, when I have done so, my support hand was up and away in a guard position protecting my head (if I'm firing from a retention position, it means my attacker is essentially on top of me). Also, I'm not willing to risk my flesh by placing it close to the cylinder gap when shooting, so no thumbs-forward grip when shooting revolvers for me.

It's all about risk/benefit.