I personally tape my controller probe to the outside of my bucket/carboy/etc with a bunch of bubble wrap over it all as an isulator.
Personal experience and actual measurement tells me that my outside temp probe reading is about 3-4 degrees lower then the center temp of my fermenting Beer; Which is why I like fermenting at 64ish (sometimes lower).. that way I'm at no more then 68 inside the fermenter. I like the way my beer comes out at that temp w/a big pitch of yeast