I use a dual display - monitor and tv outs (tv output off and monitor on - my default setting, I usually reverse this while watching films on the tv thus disabling the output to the monitor - nvidia 550Ti)

In mint 17 I was affected by an issue where any setting in displays was not being applied after rebooting. Displays always reverted to both outputs being on.

A workaround is to just create the directory /usr/share/gnome-shell/modes

sudo mkdir /usr/share/gnome-shell/modes

So far this has fixed my issue, I hope it helps someone else with a similar issue.