Quote
mikropehmo
z motor is never stepped during x or y movement anyway - or is it?
The 'spiral vase' mode of slic3r>=0.9.10 moves up slightly with each movement. I could also imagine hopping the head over existing perimeters to avoid dragging, although I don't know of any slicers that do this.
I think as long as you don't enable anything too fancy in your slicer then making this assumption should work fine. I would want to add some sort of check to the firmware to make sure an error gets printed if your gcode did have antiparallel x/z movements, rather than suddenly driving one axis the wrong way unexpectedly.