This has been discussed a few time before, and most likely a few time more; ask 3 brewers this question you will get 5 answers.
You are fermenting is a thermal mass that is exothermic.
In my opinion from what I have experienced in my brewery;
If you hang the probe free air, during the initial stages of fermenting, the wort will often be a couple of degrees warmer than the surrounding air. This will vary with the distance the probe is from the fermentor as air is a very bad conductor of heat. The more aggressive the yeast, the more this will become an issue.
If you put the probe in a jar of water, you may have a jar of water at the correct temp. Pitty about your wort, which is a larger thermal mass and is also exothermic . Therefore your wort may be warmer than the sample.
If you tape it to the side of the fermentor, and insulate it / use a probe into the wort. Then your wort is subject to the full swing of the temp controller. In this case I would set the hysteresis to 1, but at least you know the correct temp of the wort.
If you tape it to the side of the fermentor, and do not insulate it, you should get a balance of wort temp vs fridge air temp.
I have 2 fermentors in my fridge, probe stuck to the top fermentors un-insulated. I have actually measured during peak fermenting the bottom one warmer that the top one. Since coolness settles to the bottom of the fridge, this would not be expected. I can only assume that the top on being close to the cooling was exchanging the heat more effectively. I have added a 120mm pc fan to help aid in circulation and this has rectified most of the variation. This variation of temperatures within my own fridge shows that since air is not a good conductor I would not recommend either free air, nor using a sample water jar. Also if you hang your probe free air, I would ensure you have decent air circulation.
Ultimately which ever way you choose your beer is still within a temp controller environment, so will come out 100% better than a fermentor sitting on the kitchen table in the middle of summer. Think of the temperature swing that would see over a 24hour period.
QldKev