Texas - While it depends on the firearm in question, for the most part, I think most anyone would tell you a decocker is the safest way to lower a hammer. Some do remove all chances of an unintentional discharge and some don't; again, it all depends on the design of the gun. I think it's safe to say that most pistols made in the last 20 years have very safe decocking systems.
I grew up in an era of lowering a hammer by hand, and I have mastered that skill. I can remember having a hammer slip from my thumb when I was about 8 years old. My grandfather would set me in front of the TV with my Winchester 1890 and practice cocking, lowering and then setting the hammer to half cock. I did that for the better part of a year with the 1890, a Winchester 1894, a Stevens 1929 single shot shotgun and finally, the Remington 1889 double gun, which has the stiffest rebounding hammers you've ever pulled.
Anyhow, I remember the evening I let that hammer slip because I was paying more attention to the TV than I was to the gun. SNAP - I immediately looked up to see my grandfather glaring at me, and noticed his eyes divert to the muzzle of my rifle, which was, yeah you guessed it, pointing right at him. Man, talk about busted! I received a first class tongue lashing for that.
I honestly can't remember another hammer slipping from my thumb since then. It could have happened, but I really can't remember it.
With that said; if a gun has a decocker, I use it.
