Case Trimmer Wobble

The case should square up when you tighten the shellholder and pull the case rim against the lock stud.  If the wobbles, look for a burr on the flat surface of the lock stud, inside the shellholder groove, or on the case rim.  Some case wobble is tolerable.  The
case mouth will always be trimmed square because the case length gauge guides on the inside diameter of the case mouth. 
Some wobble can also be caused by the case head not being centered in the shellholder.  To minimize this, hold the case and shellholder simultaneously and spin them both with your cordless drill, gradually tightening your grip on the shellholder. 

With practice you will find you can center the case very consistently and quickly.