I never had a problem if a reporter wanted to discuss changes I made to his/her copy.
If I made a change that affected the accuracy of a story, that's a big mistake.
If I made a change that affected the tone or voice, it should have been intentional and I should be able to explain why I made the change.
In the latter case, reporters are often very defensive about their original copy, and they're often wrong; however, the discussion can be very productive. If you grasp the editor's perspective, it will help you editor-proof your stories in the future.
If the editor is defensive about changes he/she made, he/she may not be a very good editor.