I just started to do this a few days ago. I set the variable DOT_SAGENBto point to a directory in my DropBox folder, and so the Sage notebook looks in this directory for its worksheets. (I think the question you referred to dealt with the Sage notebook server running at sagenb.org, for example, not Sage running on a local machine.)
You can instead set DOT_SAGE to a directory in your DropBox folder; then Sage will look for all of its files there. (If DOT_SAGENB is not set, it defaults to DOT_SAGE, and if DOT_SAGE is not set, it defaults to $HOME/.sage.) But running Sage, and even more so, doctesting Sage, produces many temporary files in DOT_SAGE, and there is no purpose sharing those in a DropBox folder.
Edit: set the environment variable DOT_SAGENB in the file $HOME/.profile, assuming you're running bash. Add a line like
export DOT_SAGENB=~/Dropbox/sage-notebooks/
Then restart your terminal window so this file is read and executed. Then run Sage. If you want to check what Sage thinks DOT_SAGENB is set to, run this at a shell prompt:
sage -sh -c 'echo $DOT_SAGENB'
