That's the highest point in the system and will be the first place not to have liquid, rendering the sender ineffective if it gets hot and loses water.
Good point here
.. but in we're talking in theory the water temp sender shouldn't be anywhere near the radiator it should actually be on the Block itself that way when the water pump is pumping you get an accurate reading of what the engine block is the only way you get a reading on the hose side is once the thermostat opens
I would have rather tapped into the heater hoses..that way the reading is more accurate.. as it is I only start getting readings after the thermostat has truly opened