Rotatin (in general - transforming) voxels in real time is difficult compared to polygons, because with polygons you only transform vertices, and in voxels you have to transform every voxel.

EDIT: If you are spekaing about preprocessing data - yes - you can rotate as much as you want, the only problem will be that object instanced 1000 times with different orientation/scale will take 1000 times more memory than object instanced 1000 times with the same orientation/scale. With polygons there is no difference.