Well Let me say a few things. First, from what I believe soft-off is when we click the shut down and the computer goes off from there, meaning that it will not stop at the "turn off computer now" screen. This does not effect the start-up or anything else. So yes you can use it. Now ATX mobos draw power always, even when off, so if your using an inverter than you cut the power from there and you will not have a drain from the ATX board, but the drain is so small that it will not effect your cars operation. 750 milli amps off the 3.3 volt line, ( i am almost sure of that, someone correct me if they know better) is what the mobo will require when off, if using DC-DC,(with a direct line of power) and thats only because the ATX listens for the switch which is plugged into the board, and the board turns on the power supply. That holds true with or with out soft-off. Soft-off is just a feature.
Mine stays on all the time. If I am going somewhere, and stop to get gas, no porblems, or a drink, still no problems.