The safest place, IMO, is the handlebars but there are times when safety can be compromised for a small additional risk.
I have a head-mount for my main light, but it's an elastic band with a Velcro strip which holds the light above my right ear and within the outline of my helmet. It's not ideal, nothing mounted on the head can be, but the benefits of a directional light outweigh the slim chance that something mounted there would snag or hit the deck.
I've also got a couple of Knog Frogs mounted in the vents, they don't sit at all proud off the helmet's profile.
However, every 'official' helmet mount I've seen has been, again IMO, incredibly risky, offering pressure points which would transmit any impact straight to the skull, snag points to catch on branches even without the wearer falling off, or both.