Trying to align objects by manually rotating a transform is a very difficult, long, iterative process, and there is no guarantee for optimal solution, no matter how much time you spend with it.
For quick initial alignment you can use the 3D transform editing widget (in Transforms module Display/Interaction section enable “Visible in 3D view”).
For accurate alignment, we always end up using landmark registration instead of sequence of manual rotations. These modules guarantee to give you optimal solution in deterministic time, optimizing translation and rotation around all axis simultaneously, in real-time and can also compute non-rigid alignments:
- Fiducial Registration Wizard (in SlicerIGT extension): usable for any nodes (images, models, etc), does not require any initial alignment; see tutorials at www.slicerigt.org
- Landmark Registration module (in Slicer core): usable for images only, but contains nice features for defining the landmarks and setting up views; images need to be approximately aligned before
This question has been asked before, you can find some additional thoughts there: