Hi, when you disable vsync you are effectively throwing more frames at the screen, more than it can handle, hence the extra work from the GPU and heat. I think its not the best because you will face the dreadful screen tearing of vsync off.
I would use freesync if you have a capable monitor to do so and limit the fps to the maximum refresh rate of the monitor to keep things cool.
But it also depends on what you play.
Example: If you play older games, freesync can lead to strange artifacts. And if you play console ported game, usually they'll be fps locked and vsync is either not available or useless. Try lots of settings and see whats best.
As for the temps, 65-66 is a super normal reading as @FunkZ said.
The Englishman