I'm working on a manuscript. Should I save files directly to Dropbox?
There shouldn't be any issues as long as your Dropbox allotment or empty disk space on your hard drive isn't running out. Sync with your master Dropbox account will automatically take place in the background unless the network connection is turned off or is too weak.
Note that Dropbox lets you recover earlier versions of a file if the change was made in the last 30 days. You can do this via the Dropbox website. This means that should you suddenly discover the file has been corrupted, you can hopefully return to a more recent uncorrupted version of the file.
However, if the file was last changed more than 30 days ago, you'll be stuck with the most recent version -- that is, the corrupted version. For this reason I periodically save new versions of the file (usually appended with the date in the file name) in Dropbox so I will always have a slightly dated backup version to turn to, even if I haven't edited the master file in 30 days.