When making my incubator, I didn't want to increase the costs with expensive parts. So, I experimented with the dimmer switch setup for controlling temps. I found the dimmer switches to be rather inaccurate and touchy at the lower power output settings. I got much more consistent results by using the smallest light bulb I could get away with and still keep the incubator warm enough. This way, I was using the dimmer switch closer to the full power settings. The switch worked much more accurately there.
The other keys i found for getting consistent temps with a dimmer switch:
Non-fluctuating room temps. I put mine in the basement, where is seldom varies more than 2 degrees over a 3 week period.
A box with a lid that is easy to put on in the exact same position after turning eggs. If the ventilation changes, you will not be able to keep the temperature consistent.
A fan for circulation. If you use a still-air setup, temperatures are far less likely to be consistent throughout the incubator.
Not completely related to temperature, but the temperature/humidity combination is apparently also important. I found the cheap electronic hygrometers to be worthless. Readings are all over the place on the 3 different units I've tried. I made a wet-bulb thermometer and it works like a charm.
The end result is that my incubator has not varied more than a half degree with measurements taken with a medical grade themometer in a water weasel. I haven't had to fiddle with the dimmer switch. I set up the incubator 2 days early and got everything fine tuned before the eggs went in. For the first week that's gone by, it's been hassle free and accurate.