You have to know your incubator, but I recommend that you use the cheap,
glass tube fish tank thermometer from
Wal-Mart that runs a whopping $1.70. It has green suction cups. The suction cups prop the thermometer up to the level of the yolk.
If you get more than one, you can place them around the incubator to know if you have zones that vary from one another. Sticking a thermometer through the hole can kill you if you don't have the sensor
right at the level of the middle of the egg, as that height is going to be a totally different temp than what you need to measure. Even ONE INCH above the yolk height is a totally different heat than what you want to cook the eggs at, and it's irrelevant- your eggs need to be 99.5-100.5 and the other heights don't matter.
The
sealed thermometers don't have the cardboard they're glued to expanding and contracting with humidity changes, either, which can throw the whole thing off if a millimeter can make such a big difference in the temperature at which you think your eggs are cooking along.
If you know your incubator has hot spots, you can know you need to rotate your eggs in the turner occasionally. I use 3 cheapos at a time.
ETA: I've had some really nice, expensive initially accurate digital thermometers that failed as time went on because the humidity necessary during the last 3 days can toy with the electronics. They failed over time, which went unnoticed until it caused a problem.