Should you unplug the arduino power source from the outlet when you’re done cutting for the day?
Welcome, I would turn off the power to the motor shield. The arduino is powered from the USB cable from the laptop/PC/Tablet that should also be turned off. Not sure if the arduino gets power routed through the shied, in any case, I turn everything off including extensions just to be safe.
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The Arduino gets it’s power from the USB connection, so powering off the shield’s supply is safe. 
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I was wondering if the arduino get power from the shield. Not that it would do much more then finish the line(s) it has buffered if the USB gets disconnected. I could check that in the workshop but it is steam room weather here and I had enough sweat for today.
No, the Arduino gets all of it’s power from the USB connection. It would be easy to modify the shield to power the arduino (just one wire would be needed), but I chose to power it from the USB so that when you pull the USB out of your computer the machine stops immediately as a safety feature. Otherwise you end up with a robot which you can’t control doing who knows what 
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FYI - Harry added a jumper for the board powering the Arduino to the Blue Smoke Herder Board.
Thank you