I have the Nvidia driver installed in both Yosemite AND El Capitan.
But I only had reboot issues with El Capitan. So for that case, I had the driver installed, but not active (using default driver for now).
In Yosemite, the driver is installed and active. At least that's my case.
You can try adding the flag and using the default driver, instead of the proprietary Nvidia one.
Also, not sure if that makes any difference but I've got Yosemite 10.10.5, and after installing the update, I also got an update on the video drivers from Nvidia.
thanks for the idea. still not working for me. i have nvidia web driver installed, active or not, it still reboot. perhaps i'll wait for next release. thanks anyway