Nope, we've had no problems with the Latch hitting against our thumbs. I shoot Left handed a lot, but change over to Right hand at times. My Thumb folds right onto the Scooped out portion. Wife shoots Right Handed & sometimes Both Handed. She;s had no problems either. We both like the S&W Combat grips - just us. Another member posted a similar question yesterday under the heading Older S&W J Frame Grips. I posted another pic of my wife's 638 there. You've gota be careful that you don't sand into the emblem on the front nor into the back of the emblem as well. Best of luck.
Carl