On the 737s I fly the TAT probe shows correct outside temperature(if the sun ain't shining on it of course..).
Once after start up, you switch the pitot/probes heat on , the TAT indication will show you some like 99C,so obviously erroneous info.Therefore you need to use the Atis temp,vis factor,wet or dry surface to decide wether to use anti ice or not.
During the t/o roll you can notice the TAT temp decreasing as the probe is cooled by air and magic around VR it will show the outside temp.
I notice at 250 kts, TAT shows 10C, SAT shows 0c.
at 280kts ,TAT shows 10C,Sat shows -3c.....
In any case use TAT info for selecting anti ice in the air, and actual temp on the ground.
I think this is due to rime rise correction?at least on one of the turboprop i used to fly there was a correction table to be made due to rime rise factor....