The load resistor is there because the power supply regulates the 3.3V line (5V on older AT power supplies) and the other supplies are just along for the ride. IE, they are in the range because of the relative number of windings on the secondary.
But without sufficient loading, ringing occurs and while 3.3V will be regulated, the other voltages may be incorrect.
So a load resistor on the 3.3V line on an ATX supply, and a jumper on PS_ON# to turn it on.