It depends on where you have the fan mounted and what it's set to do, if the fan is perched up a cooler then it's best to have the probe between the CPU IHS(integrated heatsink) and the baseplate of the CPU cooler. If it's used as a case exhaust fan, then it's best being placed around the top of the fan where most of the heat could accumulate. If it's an intake fan then it's best to have the probe inside the case to keep track of case internal temps.