The max input voltage is 35 Volts - according to the Fairchild datasheet ( same for a lot of other manufacturers) for LM7805 thru 7018 and 40 Volts for LM7824.
And when you need less than 100mA - who cares if you waste a little bit of energy in an external voltage regulator or the regulator integrated in the Arduino.
OK, if you prefer a LM7808 ........ (what is the difference ?) - 100mW more load on the integrated regulator.
I am impressed and will change my way of thinking.