The resistor divider can be modeled as a Thevenin equivalent, so the output resistance is the equivalent resistance of the 'upper' resistance in parallel with the 'lower' resistance. Keep this value within the specified (ADC max source resistance or equiv.) range of the microcontroller and you should be fine.
Basically if the 'upper' resistor is that much larger than the 'lower' resistor, then the source resistance you assume will be very close to the 'lower' resistance value.