When decocking a revolver, rather than use just the thumb, better to pinch the tips of the support hand's index finger and thumb together, in front of the hammer nose, when decocking. This totally captures the hammer, so it is less likely to get away, until the trigger has been released. Of course, some sixguns have sharp edges there, so this may not be the most comfortable way, but it is much more sure than using just the thumb on top of a bobbed, semibobbed, or small-spurred hammer. It may also be problematic with very small weapons, especially if the user has thick thumbs and fingers.
I have practiced this, dry, with my spurless SP101 and Speed Six, and also with a bobbed-hammer S&W N-frame I used to use as a duty sixgun. I don't recall ever feeling the need to cock these weapons while they were loaded.