My heater's temperature selector rarely matches the true temperature of the tank. My water temperature in one tank is 80 degrees, which is where I want it. The number on the heater that the dial is pointing to is more like 83 or 84 degrees. I measure the temperature of the water with a digital thermometer. I use a kitchen one from the home-bed-bath type of store. I move the dial on the heater until the digital thermometer reads where I want the temp.
I have Visitherm Deluxe (glass) and Visitherm Stealth (plastic) and they have both held a steady temperature. I just don't rely on the dial to tell me the actual water temperature. As long as you can turn the dial to get the heat where you want it, and then use an accurate thermometer, you will be fine.