If you open RSLinx GUI and look under the main menu for Topic Configuration, you can see all the topics remembered by RSLinx, and any in use *should* be marked with a padlock icon. I have seen that locked status get stuck on, although the app using it had long since shut down. Only once or twice have I had to delete a topic manually but it has happened. Sometimes, I will blow all the stale ones away just to tidy up when I go in there. Using RSLogix500 Pro definitely creates a topic for every program you open offline or online. Those topics get very active when you run CGMs, big CDMs, trends and other things. I suppose it enhances responsiveness of the RSLogix interface if those DDE topics are pre-configured.
The long list of inactive topics you might find shouldn't cost you any horsepower, but I don't need a topic saved away in some mysterious harmony file for every project I might ever open and glance through.