Ok.
I have tested building with dropbox sync enabled and disabled during the build.
With dropbox sync enabled I get 60% handshake error (version 1249).
With the dropbox sync disabled I get 10% handshake error.
So its not perfect, but it helps a lot And it takes less time if you temporarly disable the dropbox sync during the build.
You can always turn the syncing back on after you have finished the build