I think it is better to just modify everything in /app/locale because you never know where an un-modified one will show up. Having modified all the ones in app/locale there is no need to actually add any new ones.
Thank you for your reply. While I agree that it is better to modify the original, I have some that were created previously. Now that I am having trouble accessing the admin interface I would like to know where the NEW files are located so my work is not lost.