I discovered that in our case a document's creator was able to edit a working copy of the document even if s/he wasn't the one who checked it out. This is because in permissionsDefinitions.xml, ROLE_OWNER has a global permission giving it FullControl. If a document doesn't have an owner property, the creator property is used.
I tried to get around this by restricting the permission for ROLE_OWNER but wasn't successful. In the end, I added a content rule to add the aspect ownable to all content. This adds a blank owner property to content and removes full control from the creator. I believe another way to do it is to add the ownable aspect to the model in configuration, but I haven't tried this. I also have to think about restricting the Take Ownership ability.
Hope this helps someone else.