I'm right handed, and if I grip my 642 (with stock Uncle Mike's boot grips) the same way as I grip all my other revolvers, I frequently get thumb gouging on the inside of my right thumb from the cylinder release. Yes, I grip my guns high up on the frame for better control / faster follow-up shots. But for some reason even when I grip the 642 in the traditional place (lower), I still get bit. I find I have to move my right thumb to the side a bit, at which point it isn't doing anything to assist with gripping the gun. I can't recall if I've ever tried putting a band-aid in the spot before you shoot, you could try that.
For me it's an issue primarily with J frames, especially the newer style latches.
Bingo, I think you may
[/I] have found a solution! Install the older style cylinder release on your 642. I'm almost certain the part is interchangeable, but others can confirm. It does have a much lower profile around the bottom edge, so it should provide more clearance to our right thumbs. I'm going to have to research that myself to see if it is feasible, it might just work, or at least make the problem less irritating.