Besides they only check a verry small surface area. the way I understand thermodynamics is the center colum and outer colums of wort are the coolest, concedering that heat always moves to a cooler surface IE the kettle and the verry top of the wort and out the top of the kettle. The best way is to have a thermo-well, that can get the overall reading of the center, middle and outter colums of the wort.
shit all that and I havent even had any beer tonight. If you are looking about doing it on the cheep like me, just get a nice long thermoater and check in different places and take the average. I hope this helped. Now it's time for a beer.