To better reflect the temperature of what's inside a fermenter, its a good idea to insulate the probe from the ambient air around the fermenter. I just tape a small piece of foam to the side of my fermenter and the probe goes next to the fermenter wall into a small groove that I cut into the foam. It might be a teeny bit off, but that's not a concern to me. Taping the probe to the fermenter wall is going to be slightly less precise, but its still going to be good enough if you're using the external air space around the fermenter to control the fermenter temperature.
If you're heating or cooling the fermenter with an internal coil or something like that, then you'll definitely want to place the probe on the fermenter wall and insulate it from the ambient air as I describe above.