One of the nice things about using a simple resistive divider network with a thermistor is that the accuracy of the excitation voltage doesn't matter because it drops out of the equation and you are solving simply for the resistance of the thermistor as a function of the resistor you place in series with it. Therefore, the accuracy is just dependent on the precision of the series resistor, the thermistor, and the ADC. A little easier to characterize over ambient temperature.
RT = RS / (1024/ADC - 1) where RT is the resistance of the thermistor and RS is the resistance of the series resistor.
If you are using the ADC from the Arduino, it is probably the least accurate link in the chain, so you can ignore the rest.