When I started I was all worried about temperature too, trying to keep it constant. After many cooks, this is such a beginners worry and frustation. Bluedawg is always right, relax.
But to answer your question, I work in instrumentation field and thermometers are so highly overrated. A thermocouple or digital thermometer is so much more accurate. When it goes bad you will know it because it will generally just fail completely. No one in the industrial field operates off of thermometers anymore. They are there for just a quick glance to see how things are going.
If you are so worried about maintaining a perfect temperature and want to remain this way, that is fine but find an automatic temperature controlling device and buy it.
XL Big Green Egg, Weber Performer Platinum, WSM 22.5"