surely if the OAT is 0 deg then any moisture forming on the pitot could potentially freeze? I was taught to check the OAT using the thermometer in the cockpit, I flew last week when it was cold and the OAT was -1 at 3,000ft, what I do is if im flying and its cloudy the pitot heater goes on, if Im flying and its marginally cold the pitot goes on.
Better to put it on than to wish you had put it on imho
D.