Published folders

When I publish the course, I've been in the habit of just publishing it into the same file folder each time.  But this got me thinking - when Storyline (or Presenter for that matter) publishes its files, does it delete the folder and start from scratch?  Or does it just copy over the files that are there?  My concerns are if its the latter - could there then be files that are left "stranded" because of changes to your presentation. 

For example - I publish a course, and it creates a file XYZ.swf.  A couple days later, I've made updates, and published again to the same folder.  Now, when it publishes, there is no longer a need for file XYZ.swf.  Is the file left there, or does the program wipe the folder before publishing?