On most Boeing aircraft other than the 737, probe heat is automatic - it comes on when you start the first engine or if air/ground says you're in-air. The probes can handle the heat although if someone forgot to remove the probe covers it could get interesting (and I suspect the 737 uses the same probes). The exception is the inlet probes on some EPR engines (747-400, 757, 767) - if you put the aircraft in air mode while on the ground (e.g. maintenance activities) you need to pull the engine probe heat breakers first of you can overheat the engine probes and damage them (and they are pricey). Used to be a common flight line problem on the 747-400 - hence new engine probe heat logic was used on the 777 and 787 EPR engines.