Is there anything that one should keep in mind when removing a model?
Or is it enough to simply delete the controller, model, and view directories that the yiic tool creates?
Do any other files get modified or anything like that?
Do you mean removing the code related to CRUD operations of a model? If so, yes it is enough to delete controller class, model class and view directory.
Actually, if you run crud, the shell should list all the new files generated by it on the screen, so you will know what files are involved.