Well... Simply forget the resistor thing (just turn off or unplug the power supply when not in use) ATX Switching mode power supplies these days are designed so that it will not be damaged as long as there is a very very little load present and since all ATX power supplies have a fan in it, a small load is always present. This makes "max power save mode" possible in which all data in RAM is saved to harddisk and basically the computer shuts down (Some ATX supplies even shuts down its own fan and still no damage) A very small additional load should wake up from this max save mode to full power via implementations like mouse break or wake-on-lan. This means charging batteries alone should wake up ATX to full power (It may not in rare cases, explained later)

Bottom line: You do not need a resistor. And if you are a worrying type, do this (even though you do not need to do this) : Hook up the batteries BEFORE you plug in the ATX to the wall and when charging is done, unplug ATX from the wall first and then take those batteries out.

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If you still cannot live without a resistor and need a heat source in cold weather, hook it up to 5v. This is because motherboard works at 5v and it is this 5v load that brings ATX supply to full power (mouse break, wake-on-lan are all 5v based) But the fan in power supply is 5v on the first place, so basically most ATX are ever-ready and you do not need a resistor in 5v line.

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In rare cases ATX power supplies will not wake up at 12v load (A very few ATXs' can't convert 12v load into 5v load) In this case, you do need a 5v load but a small one will do (resitor is not a good idea anyways since it creates lot of heat. use a led or a fan instead. CPU fans are 5v)