Judging from experience, and the forums, there is a particularly troublesome failure mode for the extruder that people tend to run into which has a tendancy to destroy motherboards/power supplies.

That issue is that eventually the insulation around the thermistor will wear through, causing one side to come in contact with the nozzle, said nozzle may actually already be shorted to one of the rails due to the heating element also wearing out, this can cause a full short which is no good.

A simple fix to keep things more robust is to incorporate another 1k or so resistor on the motherboard between ground (or 12V if that is how you have it laid out) and the thermistor. So no matter what, any short will have to go through the 1k resistor on one side, or the resistor ladder on the other, either way only a few milliamps flow and nothing lets out its magic smoke. The resistor tables may have to be adjusted slightly in the firmware but that is a small price to pay to get rid of a particularly insidious and tricky failure mode.

The thermistor won't care if you put 12v across it. the problem comes when you run 12v into the AVR's analog input, and putting 1k in the thermistor's ground leg won't change that at all. Putting it between the thermistor and the arduino would help, but would also give you an offset error due to the ADC's input impedance.

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Wooden Mendel
Teacup Firmware