I'm totally running with mortals wielding magic = hexing. Mortals not even possessing ritual or channeling, just refresh spent on other powers (psychic, were-, items of power, ect) hex only if it seems thematically appropriate to do so. A mortal with sponsorship always hexes in my games (took it off other people's advice) because the sponsor is still trying to push magic through a vessel that is not shaped to hold magic. However, in my games if you fully go over to a sponsor's side (become summer/winter knight, or equivalent, losing your humanity in the process) then hexing goes away. The exception to this is lawbreakers going towards outsiders or hell's side, who continue to hex.
I basically made my rules less on logic and more on how I wanted to emphasize humanity. If you come across Cowl, even though he may have lost free will by that point, I want it to hex because I want you to see that spark of humanity that could allow for his redemption. If you come across the summer knight, I want to convey that humanity has been lost, and thus no hexing (if they somehow reject their role, back to hexing!). If you come across an angel or demon, no hexing, but if you come across a soul-fire wielding vatican priest, hexing (to convey his humanity). A denarian won't hex unless the mortal is in charge or has been temporarily brought to the forefront (by offering the soul redemption, ect). For me, this even means that someone like Terra West (a wolf that has become human) would hex if she used evocation or thaumaturgy, as she has "gained" humanity.