When you say angle, do you mean across the face of the case neck (so if you sat it in a plate and looked accross the top it would slope, not parallel to the case head)? Or, do you mean there is a chamfer either inside/outside of the case mouth or both?
I use a Forster 3in1 trimmer that is often considered the rolls royce of trimmers (in terms of accurate cutting in any event) which does inside/outside chamfer and length, although you can adjust it slightly to reduce the chamfer.
This is usually desirable. If you cut straight onto a tube you will end up producing burrs inside and outside the tube that will need removing. A slight inner and outer chamfer also aids seating and loading, but a very sharp case mouth will take a right beating when cleaned with STM and will require regular cleaning up.
From Lee's literature.
The Deluxe Quick Trim Case Trimmer quickly and precisely trims your brass to the correct length and simultaneously chamfers the inside and outside of the case mouth.