When I bought my 637 Airweight (the clone of the 642 with a visible hammer) five years ago, I had Crimson Trace grips installed. All they had then was the 305 model, so that is what I got. If the 405s had been available, and if I knew then what I know now, I would have gotten the 405s. Even with standard pressure range loads, that sucker beats the web of my hand up. The 405 grips have that little shock absorber high up on the grips, right where I need help. I don't carry but, as I wait for a new sheriff to be elected (this is California), I practice at home. I can carry my 637, with Crimson Trace grips, in an Uncle Mike's pocket holster (thrrrrifty, and won't retain moisture) all day without printing. (I wear slacks, not jeans, so that could be a different matter) I leave the switch on all the time, even in the special place I hide it at home, because the instructions state that no current passes until the activation button is pressed. Indeed, I don't know why they have an on/off switch. As for something accidentally activating the laser in your pocket, if you have it in a name brand pocket holster, it almost certainly won't happen. And even if it does, enjoy being the center of attention.
Cordially, Jack