Note, dimensions are placed parallel to the active drawing plane, which is automatically orientated by simply moving your cursor over a face. However when the intended dimension is located away from any face you can still control the orientation by temporarily locking the drawing plane on any face that is parallel to the intended orientation of the dimension.
Select Linear Dimension tool.
Right click on a face and choose Lock Reference Plane or hit F5 key.
Click two points to create dimension and observe the dimension is parallel to the locked plane.
Be sure to Right click on the face again and choose Lock Reference Plane or hit F5 key again to unlock the plane.