[whpalmer4]

OF always keeps a local copy of your database, regardless of the sync method. You can switch between methods if you make sure to sync all your devices before switching. I would do it twice just to be sure everything was completely in sync.

When you switch things around, you'll probably get warnings about how the sync database and your local database don't appear to match. You need to be very careful about how you answer the "which one do you want to use" question. I believe OF does make a backup of your database when you switch, but I prefer not to test that portion of OF :-)

OmniFocus writes backups to the Documents/OmniFocus Backups directory regardless of your sync settings. In the OmniFocus preferences, the General pane, you'll find settings for controlling when backups are done. You may want to prune old backups occasionally, as the program doesn't do so at the moment (though it is an often-requested feature).