Are you referring to the Nudge plugin? The quintic interpolation is the default because it is accurate and relatively fast. Actually, while you are just messing around trying to get the good overlay, you could set the resampling method to the even faster linear or cubic, then when you actually "Do All", switch to Heptic (slower but more accurate than Quintic) or Fourier (even slower).
You do NOT want to nudge the +acpc and +tlrc datasets. Instead, you want to re-Talairach the +orig dataset after it was nudged. Please note that nudging will not affect the coordinates of the +orig markers, which means that you will have to re-set these markes manually using the Define Markers control panel.
