On my old Mercedes the temp. sensor is directly behind the front license plate. It is usually an acurate place, except when the car has been parked facing the sun- it goes up a few degrees. I think I'd place the sensor in front of the radiator but down low near the front plate. It would be least effected by engine temp, methinks. Also, you could get a good idea how cold the water is when you go splashing around in puddles.

I really got used to having a temp gauge in the old car and I miss it. I want one for my X!