Earthing wires should never be earthed via the transformer mounting bolts, the earth wire should have it's own bolt directly attached to the metal cabinet.
One of the reasons is that should the transformer come away from it's mount, it can leave the cabinet without an effective earth.
There are other reasons, which for the life of me I can't remember at the moment
, but this is all covered in a Standard, (which again I can't remember the number of, I'll see if I can dig it up).