Well, as I said, I don't know how Dropbox works. Or at least, how it integrates with the OS. It may be that Dropbox' /media mount actually points to somewhere else on the same disk, and so is safe to use.
But yeah, considering
how it can go boom, I'd be really careful about how I use it.
My advice would be to only keep one copy of your library (aside from backups -- always backup!), and if you want to access your books from somewhere else, use the content server(s).
I admit that it's a somewhat hypocritical advice, since while I do keep a unique library on a 24/24 Linux server, I do all my library management from a share... ^^;;