I run Dropbox (v43.4.50) on Ubuntu (16.04.3). I have unticked the "Start Dropbox on system startup" option in Dropbox Preferences but dropbox still autostarts on boot.
Even if I manually untick or remove Dropbox as a startup program in the Ubuntu Startup Applications Preferences, Dropbox still adds itself back-in the next time Dropbox is run.
This problem only came up a month or two ago so it's likely to be a bug.