Do you really feel it is relevant?
But as it happens, I don't believe it is worth it to risk my library, and yet I keep my library directly in Dropbox -- I don't regard it as risky, therefore your question is fundamentally flawed.
Nor does it matter whether Dropbox is finished syncing, if you do not modify your calibre library from multiple computers. I don't, therefore the only worry is actually having simultaneous access violations -- which is dead simple to solve in a robust, absolutely accurate manner.
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The proof is that I have done it for years and years, and never once messed up my calibre library.
So much for hasty absolutes. Did I mention I believe it wiser to offer information, then leave the choice in the hands of the questioner?
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