How to lock slices or axes to orthogonal status(be perpendicular to each other)?
By when you rotate slices using Ctrl + Alt + Left-click-and-drag, relative angle between slices planes are preserved. You can use this for quick interactive image reslicing with orthogonal planes.
You can also use Valve View module (in SlicerHeart extension). It is useful for rotation around a fixed center point.
You can also use a transform to reslice an image with orthogonal slices, using Volume Reslice Driver module (in SlicerIGT extension). It can be used in combination with Endoscopy module to reslice along a curved trajectory.
There are many more ways. If you describe what you want to do then we can give more specific advice.