That's interesting, and it disproves something I said earlier! Going back to the datasheet, it can be seen that the input voltage to the device does not change the output characteristics - it still gives 10mV / C, so that is why you don't need to change aref_voltage, and also why my earlier comment about reading the output voltage being used to feed the device and changing aref_voltage was wrong. That type of logic only applies to voltage divider types of layout.
Based on http://arduino.cc/en/Reference/AnalogRead, 5 is the correct number because that is what the input ADC handles. Because 0 - 1023 is provided for 0 - 5V, to convert the number given by AnalogRead() to voltage means TempRead * 5 / 1024.0 (as there are 1024 steps).