I use a ranco and it is set at 1 degree and I have flustuations of 4-5 degrees, At night I set it higher and during the day I set it lower, still its much easier than using a rheostat.
I think its due to the probe being directly on the UTH, if the probe was inside the enclosure I dont think I would have much fluctuation, but Im afraid if the snake pees or water is spilled on the probe then it will cook my snakes....