Depends how long the hill is and how long I am riding for.
A short rise on a quick short run and I'll be out of the saddle honking up. This is much less efficient than twiddling up the hill (your bike sways all over the place, you use energy in rocking your arms and body driving your heart rate higher than it woudl be otherwise), but can be much quicker.
But for long hills I always change early and low.