There is no correct answer, although durettd is close. Many kettles (including mine) have threaded ports on the side where you can insert your thermometer. The mash can stratify, with higher temperature at the top than the bottom, and when you stir it up there will be a big change in reading. Also, there is a delay from when the controller applies power to the heater (near the bottom) and when the temperature sensor (several inches higher) responds, and this can cause some control loops to oscillate. Using a vertical copper tube, which has excellent thermal conductivity, should help your sensor to average the temperature from top to bottom. There can still be issues with the horizontal placement, and the shape and placement of your heater coil have a major impact there. Unless your kettle is very well insulated, the outside edges will be a bit cooler than the center. I have a Blichmann boil coil, which is near the outside edge of the kettle and helps counteract that. If you have a heater that goes straight across the center then that can exaggerate the center-to-edge difference by heating the middle more than the edges.
Jut try something and see how it works. What happens when you give your mash a good stir? That will tell you how well your sensor is indicating the overall temperature of the mash. Then, even if things are not perfect, don't worry too much about it. RDWHAHB