I'm not sure whether a few of the posters in this thread actually know how a thermostat works, but it is basically an on/off switch. Most domestic fridge / freezer thermostats are pretty dumb and inaccurate. The Inkbird temperature controller with it's cooling socket is a much more accurate and smarter version of that. It performs exactly the same job as a thermostat, but it's a better one. If you adjust your fridge/freezer such that it's thermostat turns the compressor off while the Inkbird controller want it on, you have just introduced an unnecessary complication into your system. Once you task the Inkbird with controlling the temperature, you don't want the fridge/freezer thermostat to do anything except keep the compressor on. You could simply disconnect the thermostat and just wire the compressor to always on. Turning the thermostat to the coldest setting is a simpler option that will mostly achieve a similar result.
The fridge/freezer thermostat will usually measure the ambient temperature of the appliance. Most commonly there will be a capillary probe mounted somewhere on the back wall or under one of the shelves. Again, you can do better with the Inkbird controller because you have more control over where you position the probe. Positioning the probe against the object you want to cool makes more sense than measuring the temperature of the back wall.
Another factor to consider is the cooling capacity of the appliance. A fridge may have the capacity to remove enough energy to cool 25 litres of beer by 5C per hour, whereas a freezer could drop that temperature by 15C. Unless you are cold crashing, you probably want a gentler rate of change, which means multiple steps to drop the temperature. This is where the ITC-310T has an advantage over the 308. This is also where the probe positioning makes the difference.
If the probe is inside the fermenter, your compressor will keep cooling non-stop (i.e. at the fastest drop temperature rate it can achieve) until it gets to the target temperature. The difference between the fridge temperature and the beer temperature will be as big as possible until the beer reaches the target temperature. Then the cooling will stop. It is possible that due to the remaining temperature differential, the beer temperature will continue dropping and you will get overshoot.
If the probe is on the outside of the fermenter and the ambient temperature contributes partially to the readings, you are effectively measuring the "average" of the beer and fridge temperature. This means that as you start approaching the target temperature the difference between the beer temperature and fridge temperature will start getting smaller and smaller until they are the same. However, on the way there, you will reach points where the the "average" temperature is at your target and the compressor will turn off. This will result in a gentler rate of change, which is desirable as far as fermentation goes.
If you had a more sophisticated controller then taking a reading in the thermowell may well be more appropriate. If you could have multiple inputs you would also want a reading of the fridge temperature. A more sophisticated controller could allow you to control the rate of change and take into account the lag. The Inkbird controllers are not that - they are essentially just better thermostats and so you need to take the temperature at the right spot.
