There can some, but the least amount of fluctuation the better. I've never seen my Herpstat vary more than 0.3 degrees. 99.9% of the time it is dead on.
For an incubator I would buy the best thermostat you can possibly afford. Don't waste your money cheap thermostats like Hydrofarm, Zilla, ZooMed, Exo-Terra, etc. Herpstats, Helix, Ranco and the like are worth their weight in gold. If money is an issue, look for a used one.