Elimi:
You have to look at it as a system.
- What are you supplying to the sensor.
- What is your reference.
If you are only considering the built in sensors then presumably the internal reference is the same as the supply to the sensors.
If you are using external sensors, then you have to deal with the sensor as well as the reference in supplying matched voltages -- AREF to External supply -- as measured at the (analog) sensor.
Some senors like the BMA180 are more accurate when supplied with 2.4V -- go figger.
So bottom line --
- are you only talking about the built in Arduino ADC sensors?; or,
- Is this a general question? -- including things like an external Analog sensor...
I think there might be different answers depending on what you are doing...