I agree that having the RMGB files "in Dropbox" should not pose a problem. I'm a heavy Dropbox user, and I can't think of any good reason to use RM's "backup to Dropbox" option when it's so much easier just to have the RMGB files be stored in the Dropbox folder.
I also agree that it can be problematic to have the RMGC file in the Dropbox folder because any update of the RM database triggers an immediate backup of the whole database to Dropbox. This can obviously create excessive network activity. It also has been reported sometimes to cause corruption of RM databases. So a good strategy might be to keep your RMGC file outside of RM and to copy it into Dropbox only after exiting from RM. Another strategy (and the one I adopt) is to keep my RMGC file in the Dropbox folder and to pause Dropbox when I'm working in RM.
One question about best practices with RM and Dropbox has to do with why you are using Dropbox. Are you using it only as a backup mechanism, or are you using as a way to be able to use the same RM database on two different computers (not at the same time!). The strategy is much simpler if you are only using Dropbox for backup. In this case, there may not be a compelling reason ever to have your RMGC file in Dropbox provided only that you make sure that your RMGB files are in Dropbox. If ever you have a major disaster, your disaster recovery strategy would be to install RM on your new computer and the first thing you do is to restore your RMGC file on the new computer from the most recent RMGB file in Dropbox.
But if you are using Dropbox to share one RM database between two computers (not at the same time!), there are more options. For example, you could keep your RMGC file outside of Dropbox, except you copy it into Dropbox just before you switch to the other computer (and vice versa, of course). But another strategy would be never to have your RMGC file inside of Dropbox at all. Instead you keep your RMGB files in Dropbox and you always restore your RMGC file from the most recent RMGB file (no matter which computer it was created on), you always exit RM by taking a backup, and you never actually "open" the RMGC file. Instead, you always restore it at the beginning of a session and back it up at the end of a session.
So there are several good ways to use Dropbox in conjunction with RM, and also some not so good ways. You just need to choose the way that best meets your own personal needs.