Heating resistor power limit
Since they're just resistors my feeling is, I can increase the
  voltage/power and as long as it stays in it's operating temperature it
  will be fine (at least until the voltage is so high it starts to
  spark?). Is that correct?


Nearly right - you also have to consider the peak current limitations of the element and its feed wires etc.. If you drove the element with DC then you wouldn't have to worry about current; just power. However if you use a high current value for a short duration to give a moderate power (as in PWM systems) then you can still damage the device through an over-current situation rather than on total watts of heat dissipation.

Consider this transistor's (NJVMJD45H11) safe operating area graph (just a clear example of what is called the safe operating area): -



As you can see there is a thermal limit (overall power dissipation) but there's also a wire-bond limit and that is the thing that defines the maximum current. Of course there's also the voltage limit as alluded to in the question.